Merge pull request #213 from Unity-Technologies/siyaoH/1.17/fixCopy

Siyao h/1.17/fix copy

com.unity.uiwidgets/Runtime/engine/UIWidgetsPanel.cs

         }
     }
 
+    [ExecuteInEditMode]  
     public partial class UIWidgetsPanel : RawImage, IUIWidgetsWindow {
         static List<UIWidgetsPanel> panels = new List<UIWidgetsPanel>();
 
 #endif
         #endregion
 
+        IEnumerator ReEnableUIWidgetsNextFrame() {
+            yield return null;
+            enabled = true;
+        }
+
-        IEnumerator DoInitAndroid() {
-            yield return new WaitForEndOfFrame();
-            AndroidPlatformUtil.Init();
-            yield return new WaitForEndOfFrame();
-            enabled = true;
-        }
-                startCoroutine(DoInitAndroid());
+                AndroidPlatformUtil.Init();
+                startCoroutine(ReEnableUIWidgetsNextFrame());
                 return false;
             }
             return true;
 #if !UNITY_EDITOR && UNITY_ANDROID
             if (!IsAndroidInitialized()) {return ;}
 #endif
+            // If user duplicates uiwidgets gameobject in scene, canvas could be null during OnEnable, which results in error. Skip to avoid error.
+            // More explanation: during duplication, editor wakes and enables behaviors in certain order. GameObject behaviors are enabled before canvas.
+            if(canvas == null){
+                enabled = false;
+                startCoroutine(ReEnableUIWidgetsNextFrame());
+                return;
+            }
 #if !UNITY_EDITOR && UNITY_IOS
             //the hook API cannot be automatically called on IOS, so we need try hook it here
             Hooks.tryHook();
         }
 
         protected override void OnDisable() {
-            unregisterPanel(this);
-            D.assert(_wrapper != null);
-            _wrapper?.Destroy();
+            if (_wrapper != null) {
+                unregisterPanel(this);
+                _wrapper.Destroy();
+            }
+
             _wrapper = null;
             texture = null;
             Input_OnDisable();