Flutter FCM onBackgroundMessage handler gives null check error

Question

I am trying to implement notifications for my app, but when initializing notifications FirebaseMessaging.onBackgroundMessage(notificationHandler)gives error

Error:

E/flutter (28265): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: Null check operator used on a null value E/flutter (28265): #0 MethodChannelFirebaseMessaging.registerBackgroundMessageHandler (package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart:173:53) E/flutter (28265): #1 FirebaseMessagingPlatform.onBackgroundMessage= (package:firebase_messaging_platform_interface/src/platform_interface/platform_interface_messaging.dart:108:16) E/flutter (28265): #2 FirebaseMessaging.onBackgroundMessage (package:firebase_messaging/src/messaging.dart:100:31)

notificationHandler:

Future<void> notificationHandler(RemoteMessage message) async {
    /// do sth with data
}

Note: FirebaseMessaging.onMessage and FirebaseMessaging.onMessageOpenedApp works without error

Solution

It seems that you are putting FirebaseMessaging.onBackgroundMessage(notificationHandler) along with notificationHandler function within a class. However, that contradicts with the official docs of FlutterFire here. You need to fulfill these conditions for this function to work properly:

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).

My suggestion to you is that try to put the function notificationHandler that you are using as an argument outside the class that you are calling FirebaseMessaging.onBackgroundMessage() from. It worked with me.