title: Flutter: Can't use any plugin in onBackgroundMessage
body: <p>I am using Firebase Push Notifications and I want to execute some of my code when onBackgroundMessage is triggered. It actually gets triggered because I print in console, but I tried to use several plugins and no luck. I get error every time something like (Unhandled Exception: MissingPluginException(No implementation found for method play on channel flutter_ringtone_player)). I believe this is because there is no context of application in background state of application, but what is this function good for then and what can I actually do in it?</p>
  
  <p>I would like to play sound when onBackgroundMessage is triggered.</p>
  
  <pre><code>    super.initState();
  
      _firebaseMessaging.configure(
        onMessage: (Map&lt;String, dynamic&gt; message) async {
          print("onMessage: $message");
        },
        onBackgroundMessage: myBackgroundMessageHandler,
        onLaunch: (Map&lt;String, dynamic&gt; message) async {
          print("onLaunch: $message");
        },
        onResume: (Map&lt;String, dynamic&gt; message) async {
          print("onResume: $message");
        },
      );
  
  static Future&lt;dynamic&gt; myBackgroundMessageHandler(Map&lt;String, dynamic&gt; message) async {        
    FlutterRingtonePlayer.play(
      android: AndroidSounds.notification,
      ios: IosSounds.glass,
      looping: true, // Android only - API &gt;= 28
      volume: 0.8, // Android only - API &gt;= 28
      asAlarm: true, // Android only - all APIs
    );
  
      print("background message executed");
  
    return null;
  }
  </code></pre>