The argument type 'Future<dynamic> Function(String)' can't be assigned to the parameter type 'void Function(String?)?'

I am using flutter local notification but it doesn't work and it suddenly makes my app shuts down. Here's my code.

  initializeNotifications() async {
var initializationSettingsAndroid =
    AndroidInitializationSettings('@mipmap/launcher_icon');
var initializationSettingsIOS = IOSInitializationSettings();
var initializationSettings = InitializationSettings(
    android: initializationSettingsAndroid, iOS: initializationSettingsIOS);
var flutterLocalNotificationsPlugin;
await flutterLocalNotificationsPlugin.initialize(initializationSettings,
    onSelectNotification: onSelectNotification);

}

Future onSelectNotification(String payload) async {
if (payload != null) {
  debugPrint('notification payload: ' + payload);
}
await Navigator.push(
  context,
  new MaterialPageRoute(builder: (context) => MedicineReminder()),
);

}