[Flutter/dart] null check operator used on a null value messaging

Overview

A title error has occurred at FirebaseMessaging.onBackgroundMessage().

Cause and Solution

The delegate passed to the argument of the above method must be a top-level function. In my case, I was passing an instance method, so the error occurred.

The error disappeared when the method passed was a top-level function.

@main.dart

void main() async{
  //・・・
  FirebaseMessaging.onBackgroundMessage(onBackGround);
}

Future<void> onBackGround(RemoteMessage message)async{
  //・・・
}

Reference

https://firebase.flutter.dev/docs/messaging/usage/#background-messages