Flutter-Firebase Messaging Snackbar不显示

这是问题所在:应用程序在前台或后台(或已终止)都收到了应用程序发出的通知，但使用该应用程序时用于通知的小吃栏不起作用.我是新手，所以也许我犯了一些大错误.

This is the problem: notification is received by the app both with app in foreground or background (or terminated) but the snackbar for notification when the app is used doesn't work. I'm new in flutter so maybe I did some huge mistake.

我在下面显示我的代码.如果您想了解更多信息，请询问并感谢您的帮助！

I am showing my code below. If you want more information just ask and thanks for the help!

    // Import Package
    import 'dart:io';
    import 'package:app_gap_go/pages/admin/modifySingleMeeting.dart';
    import 'package:cloud_firestore/cloud_firestore.dart';
    import 'package:firebase_auth/firebase_auth.dart';
    import 'package:firebase_messaging/firebase_messaging.dart';
    import 'package:flutter/material.dart';
    // import 'package:flutter/rendering.dart';

    // Import Other Package
    import 'package:provider/provider.dart';

    // Import Pagine
    import './pages/wrapper.dart';
    import './pages/setting/settings.dart';
    import './pages/riunioni/riunioni.dart';
    import './pages/auth/register.dart';
    import './pages/admin/pannelloAdmin.dart';
    import './pages/admin/modifyMeetings.dart';

    // Import Utility
    import 'utility/themeData.dart';

    // Import Services
    import './services/authService.dart';

    // Import Models
    import './models/user.dart';

    void main() {
      // debugPaintSizeEnabled = true;   
      // debugPaintBaselinesEnabled = true;
      // debugPaintPointersEnabled = true; 
      runApp(MyApp());
    }

    class MyApp extends StatefulWidget {
      @override
      State<StatefulWidget> createState() {
        return _MyAppState();
      }
    }

    class _MyAppState extends State<MyApp> {
      final FirebaseMessaging _fcm = new FirebaseMessaging();
      final Firestore _db = Firestore.instance;
      final FirebaseAuth _auth = FirebaseAuth.instance;
      GlobalKey<ScaffoldState> scaffoldState = new GlobalKey<ScaffoldState>();

      // Request and save token
      Future _saveDeviceToken() async {
        String fcmToken = await _fcm.getToken();
        FirebaseUser user = await _auth.currentUser();
        if (fcmToken != null) {
          var tokenRef = _db
              .collection('utenti')
              .document(user.uid)
              .collection('tokens')
              .document(fcmToken);
          await tokenRef.setData({
            'token': fcmToken,
            'createdAt': FieldValue.serverTimestamp(),
            'platform': Platform.operatingSystem,
          });
        }
      }

      @override
      void initState() {
        _fcm.configure(
          onMessage: (Map<String, dynamic> message) async {
            print("onMessage: $message");

            final snackbar = SnackBar(
              content: Text(message['notification']['title']),
              action: SnackBarAction(
                label: 'Go',
                onPressed: () => null,
              ),
            );
            scaffoldState.currentState.showSnackBar(snackbar);
          },
          onResume: (Map<String, dynamic> message) async {
            print("onResume: $message");
          },
          onLaunch: (Map<String, dynamic> message) async {
            print("onLaunch: $message");
          },
        );
        if(Platform.isIOS) {
          _fcm.onIosSettingsRegistered.listen((data) {
            _saveDeviceToken();
          });
          _fcm.requestNotificationPermissions(IosNotificationSettings());
        } else {
          _saveDeviceToken();
        }
        _fcm.subscribeToTopic('meetings');
        super.initState();
      }

      @override
      Widget build(BuildContext context) {
        return StreamProvider<User>.value(
          key: scaffoldState,
          value: AuthService().user,
          child: MaterialApp(
            title: "GapGo",
            theme: themeApp,
            routes: {
              '/': (BuildContext context) => Wrapper(),
              '/register': (BuildContext context) => Register(),
              '/settings': (BuildContext context) => Settings(),
              '/riunioni': (BuildContext context) => Riunioni(),
              '/adminPanel': (BuildContext context) => PannelloAdmin(),
              '/modifyMeetings': (BuildContext context) => ModifyMeetings(),
              '/modifySingleMeeting': (BuildContext context) =>
                  ModifySingleMeeting(),
            },
          ),
        );
      }
    // End State
    }