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?

score 48 · Accepted Answer · answered May 26 '21 at 12:14

48

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.

It must not be an anonymous function.
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.

answered May 26 '21 at 12:14

Paras khandelwal

1,345
14
13

5

It was from my stupidity that `top-level` function -> put it on top of the class. – chichi May 27 '21 at 18:00
3

@chichi same stupidity here :) – swifthing Jul 24 '22 at 14:42
1

@swifthing well, looks like not just us XD – chichi Sep 07 '22 at 18:33
dont forget to add @pragma('vm:entry-point') above a function: `@pragma('vm:entry-point') Future _messageHandler(RemoteMessage message) async {}` as mentioned [here](https://firebase.google.com/docs/cloud-messaging/flutter/receive#apple_platforms_and_android) – Behzod Faiziev Feb 12 '23 at 15:13

score 20 · Answer 2 · answered Jun 14 '21 at 07:49

20

_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(
    ...
  );
}

answered Jun 14 '21 at 07:49

Rafsan Uddin Beg Rizan

1,927
18
23

1

Right. When I read about `top-level`, I assumed that I had to put it on top inside the class or function. I've realized it meant `OUTSIDE`. – chichi Jun 15 '21 at 19:24
1

Yes, put outside :) – Rafsan Uddin Beg Rizan Jun 16 '21 at 06:11

score 6 · Answer 3 · answered Jan 04 '22 at 09:43

In my case, doing what docs said was not enough. So I realized that I should add WidgetsFlutterBinding.ensureInitialized() before everything in main function like this:

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  FirebaseMessaging.onBackgroundMessage(_handleMessage);
  runApp(const Homino());
}

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

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