AK
В main.dart определи следующее:
Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
// If you're going to use other Firebase services in the background, such as Firestore,
// make sure you call `initializeApp` before using other Firebase services.
await Firebase.initializeApp();
print("Handling a background message: ${message.messageId}");
}
void main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
// other code
}

