Flutter realtime firebase is not working. No errors shown but data is not inserted into the database

I know this question have been asked before but I have tried it and still not working. That's why I post this question. Recreated the project many times and still cannot get it.

I initialize the firebase.

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

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

DatabaseReference userRef = FirebaseDatabase.instance.ref('users');

I just wanted to insert data into the realtime database with this testing code

  void _incrementCounter() {
    setState(() {
      userRef.child('testing').push().set({'testing1': 'hello', 'testing2': 'bye'});
      _counter++;
    });
  }

I have added the required file at the

Application/android/app/google-service.json

I also added the required SDK as needed for firebase. I have also tested the playground for Realtime Firebase

Playground for realtime firebase

Help is very much appreciated.