erase exception when a uipanel in prefab is being destroyed in editor, since 2018.3

Author: xiaoguzhu

Assets/Scripts/UI/UIPanel.cs

@@ -1,6 +1,9 @@
 using System;
 using System.Collections.Generic;
 using UnityEngine;
+#if UNITY_EDITOR
+using UnityEditor;
+#endif
 
 namespace FairyGUI
 {
@@ -157,7 +160,13 @@ void CreateContainer()
                 {
                     GameObject go = t.GetChild(cnt - 1).gameObject;
                     if (go.name == "UI(AutoGenerated)")
+                    {
+#if UNITY_2018_3_OR_NEWER
+                        if (PrefabUtility.IsPartOfPrefabInstance(go))
+                            PrefabUtility.UnpackPrefabInstance(PrefabUtility.GetOutermostPrefabInstanceRoot(gameObject), PrefabUnpackMode.Completely, InteractionMode.AutomatedAction);
+#endif
                         UnityEngine.Object.DestroyImmediate(go);
+                    }
                     cnt--;
                 }
             }