fix desktop platform compile error

Runtime/engine/UIWidgetsMessageManager.cs

             if (!Application.isPlaying) {
                 return;
             }
-            if (UIWidgetsMessageManager.instance != null) {
+            if (_instance != null) {
                 return;
             }
             var managerObj = new GameObject("__UIWidgetsMessageManager");
+#if UNITY_IOS || UNITY_ANDROID
-
+#endif
+        
         void OnEnable() {
             D.assert(_instance == null, "Only one instance of UIWidgetsMessageManager should exists");
             _instance = this;
         }
 
         void UpdateNameIfNeed() {
+#if UNITY_IOS || UNITY_ANDROID
-#if UNITY_IOS || UNITY_ANDROID
+
-#endif
+#endif
         }
 
         public void AddChannelMessageDelegate(string channel, MethodChannelMessageDelegate del) {

Runtime/service/keyboard.cs

             }
         }
 #endif
-#endif    
-
+#endif  
 }