prevent errors when converting UI and add warning

Author: vkovec

com.unity.formats.fbx/Editor/ConvertToNestedPrefab.cs

@@ -840,9 +840,8 @@ internal static void CopySerializedProperty(SerializedObject serializedObject, S
         }
 
         /// <summary>
-        /// Copy components on the 'from' object which is the FBX,
-        /// over to the 'to' object which is the object in the
-        /// scene we exported.
+        /// Copy components on the 'from' object which is the object being converted,
+        /// over to the 'to' object which is the FBX.
         ///
         /// Copy over everything except meshes and materials, since these
         /// are already in the FBX.
@@ -872,7 +871,10 @@ internal static void CopyComponents(GameObject to, GameObject from, GameObject r
                 }
 
                 // ignore FbxPrefab (when converting LinkedPrefabs)
-                if (fromComponent is UnityEngine.Formats.Fbx.Exporter.FbxPrefab)
+                // Also ignore RectTransform, since it is not currently possible to switch transforms
+                // in a prefab.
+                if (fromComponent is UnityEngine.Formats.Fbx.Exporter.FbxPrefab ||
+                    fromComponent is RectTransform)
                 {
                     continue;
                 }
@@ -920,6 +922,13 @@ internal static void CopyComponents(GameObject to, GameObject from, GameObject r
                     toComponent = to.AddComponent(fromComponent.GetType());
                 }
 
+                if (!toComponent)
+                {
+                    // Failed to add component
+                    Debug.LogWarningFormat("{0}: Failed to add component of type {1} to converted object", ModelExporter.PACKAGE_UI_NAME, fromComponent.GetType().Name);
+                    continue;
+                }
+
                 FixSceneReferences(fromComponent, toComponent, root);
 
                 // Do not try to copy materials for ParticleSystemRenderer, since it is not in the

com.unity.formats.fbx/Editor/ConvertToPrefabEditorWindow.cs

@@ -103,6 +103,20 @@ protected override void OnEnable()
             m_prefabExtLabelWidth = FbxExtLabelStyle.CalcSize(new GUIContent(".prefab")).x;
         }
 
+        protected bool ExportSetContainsRectTransform(out List<string> roots)
+        {
+            roots = new List<string>();
+            foreach (var obj in GetToExport())
+            {
+                var go = ModelExporter.GetGameObject(obj);
+                if (go.GetComponentInChildren<RectTransform>())
+                {
+                    roots.Add(go.name);
+                }
+            }
+            return roots.Count > 0;
+        }
+
         protected bool ExportSetContainsAnimation()
         {
             foreach (var obj in GetToExport())
@@ -143,6 +157,21 @@ protected override bool Export()
                 return false;
             }
 
+            List<string> hierarchiesWithUI;
+            if (ExportSetContainsRectTransform(out hierarchiesWithUI))
+            {
+                // Warn that UI elements will break if converted
+                string warning = string.Format("UI elements with RectTransform component present in the following hierarchies:\n\n{0}\n\nIf converted, RectTransform and other UI components may be lost",
+                    string.Join("\n", hierarchiesWithUI));
+                bool result = UnityEditor.EditorUtility.DisplayDialog(
+                    string.Format("{0} Warning", ModelExporter.PACKAGE_UI_NAME), warning, "Continue", "Cancel");
+
+                if (!result)
+                {
+                    return false;
+                }
+            }
+
             if (SettingsObject.UseMayaCompatibleNames && SettingsObject.AllowSceneModification)
             {
                 string warning = "Names of objects in the hierarchy may change with the Compatible Naming option turned on";