Firebase Firestore Error: The operator '[]' isn't defined for the class 'Object'

You are using the latest Firestore dependency, version 2.0.0, which was released May 4th, 2021 days ago. There are breaking changes, read about them here.

"Add withConverter", From the documentation:

Add withConverter function to CollectionReference, DocumentReference and Query (#6015). This new method allows interacting with collections/documents in a type-safe way:

final modelsRef = FirebaseFirestore
     .instance
     .collection('models')
     .withConverter<Model>(
       fromFirestore: (snapshot, _) => Model.fromJson(snapshot.data()!),
       toFirestore: (model, _) => model.toJson(),
     );

 Future<void> main() async {
   // Writes now take a Model as parameter instead of a Map
   await modelsRef.add(Model());
   final Model model = await modelsRef.doc('123').get().then((s) => s.data());
 }

Or, you can use simply:

String name = snapshot.data.get('name');
//instead of 
String name = (snapshot.data.data() as Map<String,dynamic>)['name']

And if you want the entire map:

Map<String,dynamic> data = snapshot.data.data() as Map<String,dynamic>);
String name = data['name'] ?? '';

Use this:

doc.get('name') ?? '', 

Instead of:

doc.data()['name'] ?? '',

First solution:

Your Brew Class:

class Brew {
  final String name;
  final String sugars;
  final int strength;

  Brew({ this.name, this.sugars, this.strength });
  
  Brew.fromJson(Map<String, dynamic> json)
      : this(
          name: json['name'] as String,
          sugars: json['sugars'] as String,
          strength: json['strength'] as int,
        );

  Map<String, Object?> toJson() {
    return {
      'name': name,
      'sugars': sugars,
      'strength': strength,
    };
  }
}

Your service:

List<Brew> _brewListCollectionReference (CollectionReference<Map<String, dynamic>> collectionReference) {
  QuerySnapshot<Object?> snapshot = await collectionReference.withConverter<Brew>(
          fromFirestore: (snapshots, _ ) => Recipe.fromJson(snapshots.data()!),
          toFirestore: (brew, _ ) => brew.toJson(),
        ).get();
  return snapshot.docs.map((doc) => doc.data() as Brew).toList();
}

Note: Your "collectionReference" is the reference from your firebase collection.

For example: firestore.collection('brewCollection')

Second solution:

List<Brew> _brewListFromSnapshot(QuerySnapshot snapshot) {
return snapshot.docs.map((doc) {
  return Brew(
    name: doc['name'] ?? '',
    strength: doc['strength'] ?? 0,
    sugars: doc['sugars'] ?? '0',
  );
}).toList();}

Third solution:

List<Brew> _brewListFromSnapshot(QuerySnapshot snapshot) {
return snapshot.docs.map((doc) {
  return Brew(
    name: doc.get('name') ?? '',
    strength: doc.get('strength') ?? 0,
    sugars: doc.get('sugars') ?? '0',
  );
}).toList();}

I discovered that Flutter now doesn't need the .data() to work properly. So change your code to represent doc['name'].

Source: The operator '[]' isn't defined for the type 'Object'. Try defining the operator '[]'