Fix call order created init + detached on dispose

Author: Ortes

lib/src/io/mobile_unity_widget_controller.dart

@@ -36,8 +36,6 @@ class MobileUnityWidgetController extends UnityWidgetController {
       unityWidgetState,
       unityId: id,
     );
-    await UnityWidgetPlatform.instance.init(id);
-    return controller;
   }
 
   void _connectStreams(int unityId) {

lib/src/io/unity_widget.dart

@@ -155,6 +155,7 @@ class _UnityWidgetState extends State<UnityWidget> {
 
   @override
   Future<void> dispose() async {
+    widget.onUnityDetached?.call();
     if (!kIsWeb) {
       if (_nextUnityCreationId > 0) --_nextUnityCreationId;
     }

lib/src/web/web_unity_widget_controller.dart

@@ -35,6 +35,9 @@ class WebUnityWidgetController extends UnityWidgetController {
 
   MethodChannel? _channel;
 
+  @override
+  int get id => throw UnimplementedError("Platform does not support id");
+
   /// used for cancel the subscription
   StreamSubscription? _onUnityMessageSub,
       _onUnitySceneLoadedSub,