Flutter: FCM Unhandled Exception: Null check operator used on a null value

Question

E/flutter (26872): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value E/flutter (26872): #0
MethodChannelFirebaseMessaging.registerBackgroundMessageHandler (package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart:173:53) E/flutter (26872): #1
FirebaseMessagingPlatform.onBackgroundMessage= (package:firebase_messaging_platform_interface/src/platform_interface/platform_interface_messaging.dart:108:16)

// Background Messaging Set Up
    Future<void> _firebaseMessagingBackgroundHandler(
        RemoteMessage message) async {
      print('background message');
    }

    FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
    runApp(....)

I am getting an error for this code on Android system. Everything works except when the app is terminated.

What works on Android:

• Notification when Terminated, onBackground and onForeground
• Date only when onForeground

What does not work on Android:

• Data only when Terminated and onBackground

What works on iOS:

• Notification when Terminated, onBackground and onForeground
• Date only when onForeground

What does not work on iOS:

• Data only when Terminated,

I have no clue why I am getting that null value error on Android system and how can I fix this issue? Also, is it true that I can not receive the Data only push notification on iOS when the app is terminated?

Answer 1

I had the same error as like you, on the same line. I checked out docs and it says 2 things about background message handler.

1. It must not be an anonymous function.
2. It must be a top-level function (e.g. not a class method which requires initialization).

In my case it was not a top-level function, it was declared inside a class. When you move your handler out of any class or function, so that it is a top-level function and doesn't require any class or method initialisation then the error will be gone.

Answer 2

_firebaseMessagingBackgroundHandler function should be outside of the main function.

Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  await Firebase.initializeApp();
}


Future<void> main() async {

  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);


  runApp(
    ...
  );
}