initialize the flutter_local_notification plugin

notification_api.dart

class NotificationApi{
  static final _notification = FlutterLocalNotificationsPlugin();

  static void init() {
    _notification.initialize(
      const InitializationSettings(
        android: AndroidInitializationSettings('@mipmap/ic_launcher'),
        iOS: DarwinInitializationSettings(),
      ),
    );
  }
}