Starting Since August 17 2020

All Firebase versions have been updated and now you have to call Firebase.initializeApp() before using any Firebase product, for example:

First, all Firebase products now depend on firebase_core version (0.5.0+), therefore you need to add it in the pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  firebase_core : ^0.5.0
  # cloud_firestore: ^0.14.0 other firebase dependencies

Then you have to call Firebase.initializeApp():

First Example

import 'package:flutter/material.dart';

// Import the firebase_core plugin
import 'package:firebase_core/firebase_core.dart';

void main() {
  runApp(App());
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      // Initialize FlutterFire
      future: Firebase.initializeApp(),
      builder: (context, snapshot) {
        // Check for errors
        if (snapshot.hasError) {
          return SomethingWentWrong();
        }

        // Once complete, show your application
        if (snapshot.connectionState == ConnectionState.done) {
          return MyAwesomeApp();
        }

        // Otherwise, show something whilst waiting for initialization to complete
        return Loading();
      },
    );
  }
}

Second Example with Firestore:

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: FirstRoute(title: 'First Route'),
    );
  }
}

class FirstRoute extends StatefulWidget {
  FirstRoute({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _FirstRouteState createState() => _FirstRouteState();
}

class _FirstRouteState extends State<FirstRoute> {
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("test"),
        ),
        body: FutureBuilder(
          future: getData(),
          builder: (context, AsyncSnapshot<DocumentSnapshot> snapshot) {
            if (snapshot.connectionState == ConnectionState.done) {
              return Column(
                children: [
                  Container(
                    height: 27,
                    child: Text(
                      "Name: ${snapshot.data.data()['name']}",
                      overflow: TextOverflow.fade,
                      style: TextStyle(fontSize: 20),
                    ),
                  ),
                ],
              );
            } else if (snapshot.connectionState == ConnectionState.none) {
              return Text("No data");
            }
            return CircularProgressIndicator();
          },
        ));
  }

  Future<DocumentSnapshot> getData() async {
    await Firebase.initializeApp();
    return await FirebaseFirestore.instance
        .collection("users")
        .doc("docID")
        .get();
  }
}

Third Example:

Initialize it in initState() then call setState() which will call the build() method.

  @override
  void initState() {
    super.initState();
    Firebase.initializeApp().whenComplete(() { 
      print("completed");
      setState(() {});
    });
  }

Fourth Example:

Initialize it in the main() method after calling WidgetsFlutterBinding.ensureInitialized();

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

Note: You only have to call initializeApp() once

1. Add to pubspec.yaml

   firebase_core :
   

2. add to main.dart

   import 'package:firebase_core/firebase_core.dart';
   
   void main() async {
      WidgetsFlutterBinding.ensureInitialized();
      await Firebase.initializeApp();
      runApp(MyApp());
   }
   

If you followed Peter's answer and are still getting the same error, check to make sure anything else you have in your main function comes after the await Firebase.initializeApp() call, like so:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  FirebaseCrashlytics.instance.setCrashlyticsCollectionEnabled(true);
  FlutterError.onError = FirebaseCrashlytics.instance.recordFlutterError;
  runApp(MyApp());
}

Since August 2017, all Firebase services have been updated so that you have to call Firebase.initializeApp() in your main before you can use any of the Firebase products, for example:

If you want to use the firebase_core plugin in a flutter application, then in your pubspec.yaml file add the firebase_core as below

dependencies:
  flutter:
    sdk: flutter
  firebase_core: ^1.19.1

Then call the Firebase.initializeApp(): in your app. same as something below:

import 'package:firebase_core/firebase_core.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

First, add this dependency:

firebase_core :

Second: in the project main function, add these two lines and make the function async:

void main() async {
    // These two lines
    WidgetsFlutterBinding.ensureInitialized();
    await Firebase.initializeApp();

    //
    runApp(MyApp());
}

Now you can use Firebase normally in any file or widget in the project.

The FutureBuilder widget will work either, but you must add it every time you want to access Firebase.

Dart-only initialization (Recommended)

• Initialize Firebase on all platforms using CLI.

• No need to add any native code.

• No need to download any Google services file.

1. Install FlutterFire CLI

   dart pub global activate flutterfire_cli
   

2. (Optional) Add the following line to your ~/.zshrc file and save it

   export PATH="$PATH":"$HOME/.pub-cache/bin"
   

3. Run the configure:

   flutterfire configure
   

4. Press enter to configure all the platforms

   

5. You should now have a lib/firebase_options.dart file. Import this file in your main.dart and use Firebase.initializeApp.

   import 'firebase_options.dart';
   
   void main() async {
     WidgetsFlutterBinding.ensureInitialized();
   
     // This is the last thing you need to add. 
     await Firebase.initializeApp(
       options: DefaultFirebaseOptions.currentPlatform,
     );
   
     runApp(...);
   }
   

Migrating to Dart-only initialization:

If you already have an existing application, you can migrate it in the following ways.

• Android:

  1. Delete google-services.json file from <flutter-project>/android/app:

     

  2. Delete the following line in <flutter-project>/android/build.gradle file

     

  3. Delete the following line in <flutter-project>/android/app/build.gradle file

     

• iOS

  Open <flutter-project>/ios/Runner.xcworkspace file in Xcode and delete the GoogleService-Info.plist file from the Runner.

  

• macOS

  Open <flutter-project>/macos/Runner.xcworkspace file in Xcode and delete the GoogleService-Info.plist file from the Runner.

  

More information can be found here

Peter's answer is perfect!! But if you still get an error in your code and following the Flutter Firebase codelab, note that those tutorials are outdated as of August 2020 and not updated yet.

You need to do many other changes like:

• replace .data with .data()
• replace updateData with update

Flutter Web with Firebase

If you are using Flutter Web with Firebase make sure you have the SDKs installed in the body tag of your ./web/index.html

  <script src="https://www.gstatic.com/firebasejs/8.2.9/firebase-app.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.2.9/firebase-analytics.js"></script>

Furthermore, make sure you call firebase.initializeApp(...) with the configuration parameters in the index.html as well.

  <script>
    // Your web app's Firebase configuration
    // For Firebase JS SDK v7.20.0 and later, measurementId is optional
    var firebaseConfig = {
      apiKey: "AIz...",
      authDomain: "...",
      databaseURL: "https://<project-name>.firebaseio.com",
      projectId: "...",
      storageBucket: "<project-name>.appspot.com",
      messagingSenderId: "...",
      appId: "...",
      measurementId: "..."
    };
    firebase.initializeApp(firebaseConfig);
    firebase.analytics();
  </script>

At last configure firebase as described in Peter Haddad's answer:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

You need to add await Firebase.initializeApp(); which is a Future. Do it inside the dart file that is running your Firebase function like below:

import 'package:firebase_core/firebase_core.dart';
...

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MaterialApp());
}

If you still have the problem when you leave the app to the main screen, you could add this to whatever .dart file using Firebase:

class App extends StatelessWidget {

  final Future<FirebaseApp> _initialization = Firebase.initializeApp();

  @override
  Widget build(BuildContext context) {

Or in the case of a StatefulWidget:

import 'package:flutter/material.dart';

// Import the firebase_core plugin
import 'package:firebase_core/firebase_core.dart';

void main() {
  runApp(App());
}

class App extends StatefulWidget {
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  // Set default `_initialized` and `_error` state to false
  bool _initialized = false;
  bool _error = false;

  // Define an async function to initialize FlutterFire
  void initializeFlutterFire() async {
    try {
      // Wait for Firebase to initialize and set `_initialized` state to true
      await Firebase.initializeApp();
      setState(() {
        _initialized = true;
      });
    } catch(e) {
      // Set `_error` state to true if Firebase initialization fails
      setState(() {
        _error = true;
      });
    }
  }

  @override
  void initState() {
    initializeFlutterFire();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    // Show error message if initialization failed
    if(_error) {
      return SomethingWentWrong();
    }

    // Show a loader until FlutterFire is initialized
    if (!_initialized) {
      return Loading();
    }

    return MyAwesomeApp();
  }
}

For more information, check this link.

Since Flutter versions 2.8, FlutterFire can now be initialized from Dart on all platforms using the Firebase.initializeApp function. There's a more simple and quicker way to initialize a firebase project, instead of downloading the .json and .plist file from the firebase console and placing them into your Flutter project folders which can result in several warnings. This is a problem I've experienced several times and here's how I solved it.

After creating your project in the firebase console steps here and installed the Firebase CLI here, please proceed with the following steps:

You can skip step 1 and jump to step 4 if you've already configured your firebase project with flutterfire configure.

1. From your project root terminal, command:

     $ flutterfire configure   
       // This requires the Firebase CLI to work.
   

2. Select firebase project by hitting return or enter.

3. insert the bundle ID. You can get this by right clicking the ios folder > then click'Open in xcode'. The Bundle Identifier will appear in the right white panel and you'll have to type this in the terminal manually when prompted.

** Proceed to step 5 if you already have the firebase_core plugin installed. **

4. Install the latest version of the firebase_core plugin by running this command from your project root directory:

    $ flutter pub add firebase_core  // Adds to pubspec.yaml
   

5. Add imports to the main file:

    import 'package:firebase_core/firebase_core.dart'; // 
    import 'firebase_options.dart'; // Generated file
   

6. Update your main function to initialize firebase with this async function:

     Future<void> main() async {
      WidgetsFlutterBinding.ensureInitialized();
      await Firebase.initializeApp(options: 
      DefaultFirebaseOptions.currentPlatform);
      runApp(const YourAppName());
     }
   

See the FlutterFire documentation for more information.

This is the way to initialize firebase in 2022

First of all, Install FlutterFire with this link ==> Flutter Fire Installation Link

After installing FlutterFire, you'll need to add these lines of code in your lib/main.dart file

  import 'package:firebase_core/firebase_core.dart';
  import 'firebase_options.dart';  

Then edit your Main Function as done below

void main() async {
 WidgetsFlutterBinding.ensureInitialized();
   await Firebase.initializeApp(options:                                  
   DefaultFirebaseOptions.currentPlatform,);
 runApp(MyApp()); 
 }

This can change in the future, so refer to this link for the latest implementation reference below

Initializing FlutterFire#

I needed to reconfigure the Firebase app with FlutterFire cli.

https://firebase.flutter.dev/docs/cli

1. Install firebase-tool via NPM: npm install -g firebase-tools
2. Install flutterfire cli: dart pub global activate flutterfire_cli
3. Run this command to configure firebase in your project: flutterfire configure

Follow the above link and initialise as follows:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
  runApp(const MyApp());
}

As its await call, The app will show splash screen and Firebase may not initialise so I proceeded next with another FutureBuilder for home screen to make sure Firebase app is initialised before I use FirebaseAuth.

home: FutureBuilder(
          future: Firebase.initializeApp(),
          builder: (context, snapshot) {
            if (snapshot.hasError) {
              return Text(snapshot.error.toString(),
                  style: TextStyle(fontSize: 10));
            }

            if (snapshot.connectionState == ConnectionState.done) {
              return StreamBuilder(
                stream: FirebaseAuth.instance.authStateChanges(),
                builder: (context, snapshot) {
                  return snapshot.hasData ? Home() : AuthPage();
                },
              );
            } else {
              return Text('');
            }
          })

You can initialize in the main.dart file as well like this using FutureBuilder.

 future: Firebase.initializeApp(),

Look at this example.

void main() {
  runApp(App());
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      // Initialize FlutterFire
      future: Firebase.initializeApp(),
      builder: (context, snapshot) {
       return MaterialApp(
          debugShowCheckedModeBanner: false,
          title: 'NEWSAPI.org',
          home: SplashScreen(),
       
          routes: <String, WidgetBuilder>{
            SPLASH_SCREEN: (BuildContext context) => SplashScreen(),
          },);
      },
    );
  }
}

you neeed to add await FirebaseApp.initializeApp() in main function remember to add await in main function

If you've set Firebase correctly, do not forget to add

await

before

Firebase.initializeApp();

That is mainly the problem with that Exception.

await Firebase.initializeApp();

On my case, I added web support to my existing project. The one thing which solved the issue is,

In the web/index.html file

Changing the verison of all the firebase sdk imports from 9.6.6 to 8.10.0

Snippet

<script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-app.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-app-check.js"></script>  
  <script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-firestore.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-auth.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-storage.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.10.0/firebase-messaging.js"></script>
 <script>
  var firebaseConfig = {
   ....
   }

Always remember in your mind that firebase is initialized only once in your project so you use in main class because main class is called only once if application is open

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

In most cases this problem is observed after upgrading the project from older Flutter version to null safety. These things should be checked:

Ensure you have firebase-core in pubscpec.yaml

Import firebase core:

import 'package:firebase_core/firebase_core.dart';

Init Firebase before any call - preferably inside main.dart so it is executed before any other implementation:

WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp(
  options: DefaultFirebaseOptions.currentPlatform,
);

From offical documentation.

In my case it was because for the Web version you have to manually add stuff in the index.html file, not only the .js dependency, but the configuration. See Web Installation.

We tend to forget that for most stuff Flutter is compiled to the target without the need to change anything target-specific, but in this case one must add the configuration for each target.

Use This for flutter :

   const firebaseConfig = {

    apiKey: "",
    authDomain: "",
    projectId: "",
    storageBucket: "",
    messagingSenderId: "",
    appId: "",
    measurementId: ""
    };
   
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);
  firebase.getAnalytics(app);

Instead of this

   const firebaseConfig = {

    apiKey: "",
    authDomain: "",
    projectId: "",
    storageBucket: "",
    messagingSenderId: "",
    appId: "",
    measurementId: ""
    };

    // Initialize Firebase
    const app = initializeApp(firebaseConfig);
    const analytics = getAnalytics(app);

I wanted to comment. But this issue have lot of trails. For my case.

I was initializing Injection Container Before Firebase. Hierarchy needs to be check.

Hive.init(directory.path);
  await Firebase.initializeApp();// before
  await di.init();

Initialise with firebase options

void main() async{
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp(
options: DefaultFirebaseOptions.currentPlatform).catchError((e){
  print(" Error : ${e.toString()}");
});
}

like this

If you are working on a project in which the firebase_core was preconfigured and then you are trying to add new packages it will give that error.

You have to first remove firebase_core: from pubsec.yaml and then save the file and reconfigure it through flutter pub add firebase_core and then flutterfire configure.

Then you can add any new packages.

Initialise with firebase options

void main() async{
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(
  options: DefaultFirebaseOptions.currentPlatform).catchError((e){
    print(" Error : ${e.toString()}");
  });
}

The problem for me was I initialized it without "await":

void main() async {
    WidgetsFlutterBinding.ensureInitialized();
    Firebase.initializeApp();

The correct way is:

void main() async {
    WidgetsFlutterBinding.ensureInitialized();
    await Firebase.initializeApp();