flutter - Flutter の workmanager で sharedPreferences のパラメータを使用するにはどうすればよいですか?

Question

ユーザーがsharedPreferencesに保存し、URLに追加して呼び出しを行うパラメーターに基づいて通知を送信しようとしています。savedCity を作成すると通知が機能しますが、sharedPreferences から取得しようとすると失敗します。どうすればこれを機能させることができますか？

void callbackDispatcher() {
  Workmanager.executeTask((task, inputData) async {
    String savedCity = "";

 //    final prefs = await SharedPreferences.getInstance();
 //     savedCity = prefs.getString('defaultCity') ?? ""; 

    if (task == 'uniqueKey') {
      var response = await http.get(Uri.parse('http://api.openweathermap.org/data/2.5/weather?q=$savedCity&units=metric&appid=$API_KEY'));
      Weather dataComingFromTheServer = Weather.fromJson(json.decode(response.body));

      final FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
      FlutterLocalNotificationsPlugin();
      const AndroidNotificationDetails androidPlatformChannelSpecifics =
      AndroidNotificationDetails('your channel id', 'your channel name',
          'your channel description',
          importance: Importance.max,
          priority: Priority.high,
          showWhen: false);
      const NotificationDetails platformChannelSpecifics =
      NotificationDetails(android: androidPlatformChannelSpecifics);
      await flutterLocalNotificationsPlugin.show(
          0,
          dataComingFromTheServer.name,
          '${dataComingFromTheServer.weather[0].description} ${dataComingFromTheServer.main.tempMin}ºC - ${dataComingFromTheServer.main.tempMax}ºC',
          platformChannelSpecifics,
          payload: 'item x');
    }
    return Future.value(true);
  });
}

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
  FlutterLocalNotificationsPlugin();
  const AndroidInitializationSettings initializationSettingsAndroid =
  AndroidInitializationSettings(
      '@mipmap/ic_launcher');
  final IOSInitializationSettings initializationSettingsIOS =
  IOSInitializationSettings();
  final MacOSInitializationSettings initializationSettingsMacOS =
  MacOSInitializationSettings();
  final InitializationSettings initializationSettings = InitializationSettings(
      android: initializationSettingsAndroid,
      iOS: initializationSettingsIOS,
      macOS: initializationSettingsMacOS);
  await flutterLocalNotificationsPlugin.initialize(initializationSettings,
      onSelectNotification: selectNotification);

  Workmanager.initialize(
      callbackDispatcher,
      isInDebugMode:
      true);

  Workmanager.registerPeriodicTask(
    "1",
    "uniqueKey",
    frequency: Duration(minutes: 15),
  );
  runApp(MyApp());
}

void setPreference(String cityName) async {
    final prefs = await SharedPreferences.getInstance();

    prefs.setString('defaultCity', cityName);

}