我正试图通过遵循youtube中的代码使用FCM从firebase推送通知。当运行系统时，我发现这个错误：
[错误：flutter/lib/ui/ui_dart_state.cc（199）]无法处理的异常：PlatformException（error，NotificationSystem. Group不存在，null，java.lang.IllegalArgumentException：通知用户组不存在
在firebase控制台中，状态是完成，但它没有推送到模拟器。有谁能帮帮我有什么问题吗？
主.dart代码

import 'package:flutter/material.dart';

import 'package:flutter_localizations/flutter_localizations.dart';
import 'package:monger_app/WelcomeScreen/login.dart';
import 'package:monger_app/WelcomeScreen/signup.dart';
import 'package:monger_app/WelcomeScreen/welcome_screen.dart';
import 'package:monger_app/localization/demo_localization.dart';
import 'package:monger_app/page/pushNotification.dart';
import 'package:monger_app/page/root.dart';
import 'package:monger_app/theme/colors.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:provider/provider.dart';
import 'localization/localization_constants.dart';


void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  //FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

  return runApp(ChangeNotifierProvider(
    child: MyApp(),
    create: (BuildContext context) => ThemeProvider(isDarkMode: true),
  ));
}

class MyApp extends StatefulWidget {

  static void setLocale(BuildContext context, Locale locale){
    _MyAppState state = context.findAncestorStateOfType<_MyAppState>();
    state.setLocale(locale);
  }
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  Locale _locale;

  void setLocale(Locale locale){
    setState(() {
      _locale = locale;
    });
  }

  FirebaseNotifcation firebase;

  handleAsync() async {
    await firebase.initialize();

    String token = await firebase.getToken();
    print("Firebase token : $token");
  }

  @override
  void initState() {
    super.initState();
    firebase = FirebaseNotifcation();
    handleAsync();
  }

 @override
  void didChangeDependencies() {
    getLocale().then((locale){
      setState(() {
        this._locale = locale;
      });
    });
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    return Consumer<ThemeProvider>(
      builder: (context, themeProvider, child){
        if (_locale == null) {
          return Container(
            child: Center(
              child: CircularProgressIndicator(),
            ),
          );
        } else {
          return MaterialApp(
            theme: themeProvider.getTheme,
            locale: _locale,
            supportedLocales: [
              Locale('en', 'US'),
              Locale('id', 'ID'),
              Locale('zh', 'CN'),
            ],
            localizationsDelegates: [
              DemoLocalization.delegate,
              GlobalMaterialLocalizations.delegate,
              GlobalWidgetsLocalizations.delegate,
              GlobalCupertinoLocalizations.delegate,
            ],
            localeResolutionCallback: (deviceLocale, supportedLocales){
              for (var locale in supportedLocales) {
                if (locale.languageCode == deviceLocale.languageCode && locale.countryCode == deviceLocale.countryCode) {
                  return deviceLocale;
                }
              }
              return supportedLocales.first;
            },
            debugShowCheckedModeBanner: false,
            home:

            WelcomeScreen(),

            routes: <String,WidgetBuilder>{

              "SignIn" : (BuildContext context)=>LoginPage(),
              "SignUp":(BuildContext context)=>SignUpPage(),
              "start":(BuildContext context)=>Root(),
              "welcome":(BuildContext context)=>WelcomeScreen(),
            },

          );
        }
      }
    );
  }
}

puchNotification.dart代码

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_messaging/firebase_messaging.dart';
import 'package:flutter_local_notifications/flutter_local_notifications.dart';

const AndroidNotificationChannel channel = AndroidNotificationChannel(
    'high_importance_channel',
    "High Importance Notifcations",
    "This channel is used important notification",
    groupId: "Notification_group");

final FlutterLocalNotificationsPlugin flutterLocalNotificationplugin =
FlutterLocalNotificationsPlugin();

Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  await Firebase.initializeApp();
  print("Handling a background message : ${message.messageId}");
  print(message.data);
}

class FirebaseNotifcation {
  initialize() async {
    await Firebase.initializeApp();
    FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

    await flutterLocalNotificationplugin
        .resolvePlatformSpecificImplementation<
        AndroidFlutterLocalNotificationsPlugin>()
        ?.createNotificationChannel(channel);
    var intializationSettingsAndroid =
    AndroidInitializationSettings('@mipmap/ic_launcher');
    var initializationSettings =
    InitializationSettings(android: intializationSettingsAndroid);

    flutterLocalNotificationplugin.initialize(initializationSettings);
    FirebaseMessaging.onMessage.listen((RemoteMessage message) async {
      RemoteNotification notification = message.notification;
      AndroidNotification android = message.notification?.android;
      if (notification != null && android != null) {
        AndroidNotificationDetails notificationDetails =
        AndroidNotificationDetails(
            channel.id, channel.name, channel.description,
            importance: Importance.max,
            priority: Priority.high,
            groupKey: channel.groupId);
        NotificationDetails notificationDetailsPlatformSpefics =
        NotificationDetails(android: notificationDetails);
        flutterLocalNotificationplugin.show(
            notification.hashCode,
            notification.title,
            notification.body,
            notificationDetailsPlatformSpefics);
      }

      List<ActiveNotification> activeNotifications =
      await flutterLocalNotificationplugin
          .resolvePlatformSpecificImplementation<
          AndroidFlutterLocalNotificationsPlugin>()
          ?.getActiveNotifications();
      if (activeNotifications.length > 0) {
        List<String> lines =
        activeNotifications.map((e) => e.title.toString()).toList();
        InboxStyleInformation inboxStyleInformation = InboxStyleInformation(
            lines,
            contentTitle: "${activeNotifications.length - 1} messages",
            summaryText: "${activeNotifications.length - 1} messages");
        AndroidNotificationDetails groupNotificationDetails =
        AndroidNotificationDetails(
            channel.id, channel.name, channel.description,
            styleInformation: inboxStyleInformation,
            setAsGroupSummary: true,
            groupKey: channel.groupId);

        NotificationDetails groupNotificationDetailsPlatformSpefics =
        NotificationDetails(android: groupNotificationDetails);
        await flutterLocalNotificationplugin.show(
            0, '', '', groupNotificationDetailsPlatformSpefics);
      }
    });
  }

  Future<String> getToken() async {
    String token = await FirebaseMessaging.instance.getToken();
    print(token);
    return token;
  }

  subscribeToTopic(String topic) async {
    await FirebaseMessaging.instance.subscribeToTopic(topic);
  }
}

build.gradle文件

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"

android {
    compileSdkVersion 30

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.example.monger_app"
        minSdkVersion 21
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
}

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation platform('com.google.firebase:firebase-bom:28.0.1')
    implementation "com.google.firebase:firebase-messaging:20.1.0"
}

apply plugin: 'com.google.gms.google-services'

AndroidManifest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.example.monger_app">

   <application
       tools:replace="android:label"
        android:name=".Application"
        android:label="MONGE"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <!-- Specifies an Android theme to apply to this Activity as soon as
                 the Android process has started. This theme is visible to the user
                 while the Flutter UI initializes. After that, this theme continues
                 to determine the Window background behind the Flutter UI. -->
            <meta-data
              android:name="io.flutter.embedding.android.NormalTheme"
              android:resource="@style/NormalTheme"
              />
            <!-- Displays an Android View that continues showing the launch screen
                 Drawable until Flutter paints its first frame, then this splash
                 screen fades out. A splash screen is useful to avoid any visual
                 gap between the end of Android's launch screen and the painting of
                 Flutter's first frame. -->
            <meta-data
              android:name="io.flutter.embedding.android.SplashScreenDrawable"
              android:resource="@drawable/launch_background"
              />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
            <intent-filter>
                <action android:name="FLUTTER_NOTIFICATION_CLICK" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
        <!-- Don't delete the meta-data below.
             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
        <meta-data
            android:name="flutterEmbedding"
            android:value="2" />
    </application>
</manifest>