Revert "fixed null-aware"

Author: kalismeras61

example/lib/generated_plugin_registrant.dart

@@ -4,7 +4,6 @@
 
 // ignore_for_file: directives_ordering
 // ignore_for_file: lines_longer_than_80_chars
-// ignore_for_file: depend_on_referenced_packages
 
 import 'package:assets_audio_player_web/web/assets_audio_player_web.dart';
 

lib/src/assets_audio_player.dart

@@ -520,7 +520,7 @@ class AssetsAudioPlayer {
     _playerEditor = null;
 
     if (_lifecycleObserver != null) {
-      WidgetsBinding.instance.removeObserver(_lifecycleObserver!);
+      WidgetsBinding.instance?.removeObserver(_lifecycleObserver!);
       _lifecycleObserver = null;
     }
   }
@@ -690,7 +690,7 @@ class AssetsAudioPlayer {
       }
     });
     if (_lifecycleObserver != null) {
-      WidgetsBinding.instance.addObserver(_lifecycleObserver!);
+      WidgetsBinding.instance?.addObserver(_lifecycleObserver!);
     }
   }