Flutter: Fatal Error: Callback lookup failed! (with audioplayers package)

I faced the same issue, so this works for my game code, I hope this works for you, first, create a Controller Class like this:

import 'package:audioplayers/audio_cache.dart';
import 'package:audioplayers/audioplayers.dart';

void audioPlayerHandler(AudioPlayerState value) => print('state => $value');

class GameController {

  static AudioPlayer audioPlayer = AudioPlayer();
  static AudioCache audioCache = AudioCache();

  static void play(String sound) {
    if (!kIsWeb && Platform.isIOS) {
      audioPlayer.monitorNotificationStateChanges(audioPlayerHandler);
    }
    audioCache.play(sound);
  }
}

Then use your code by this way:

FlatButton(
  onPressed: () => {
    GameController.play('note1.wav');
  }
  child: Text('Click to ding!'),
),