[Solved] No Firebase App ’[DEFAULT]’ has been created - call Firebase.initializeApp() Error in Flutter

While using Firebase services in Flutter, you may get "No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()" Error in Flutter. This error is caused when you use any Firebase services such as Cloud Messaging, Firestore without initializing Firebase core.

Error Message:

E/flutter (10040): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] 
Unhandled Exception: [core/no-app] No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()
E/flutter (10040): #0      MethodChannelFirebase.app 
(package:firebase_core_platform_interface/src/method_channel/method_channel_firebase.dart:149:5)
E/flutter (10040): #1      Firebase.app (package:firebase_core/src/firebase.dart:55:41)
E/flutter (10040): #2      FirebaseMessaging.instance (package:firebase_messaging/src/messaging.dart:32:47)

Here, in our case, this error occurs while using Firebase cloud messaging.

Cause of Error:

The cause of this error is you might not have initialized firebase before using firebase service, or you may have initialized like below:

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

  FirebaseMessaging.instance.subscribeToTopic("all"); 
  //use of Firebase service without initialization of Firebase core.
  
  runApp(MyApp());

}

OR

void main() async {
   WidgetsFlutterBinding.ensureInitialized();
  Firebase.initializeApp(); //initilization of Firebase app
  // here, Firebase.initilizeApp() is Future method, so you need to add await before.
  //run time error: Unhandled Exception: [core/no-app] 
  //No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()
  
  runApp(MyApp());
}

How to solve this error?

To solve this error, add firebase_core Flutter package in your project by adding the following lines in pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  firebase_core: ^1.10.6

Now in main() method of main.dart file, initialize Firebase like below before using any Firebase services:

import 'package:firebase_core/firebase_core.dart';

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

  await Firebase.initializeApp();
   //initilization of Firebase app
  
  // other Firebase service initialization

  runApp(MyApp());
}

Here, we have initialized Firebase using Firebase.initializeApp() method. Remember, this is a Future method, so don't forget to ass "await" before it, otherwise, you will encounter the same error.

In this way, you can solve "No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()" error in the Flutter app.