Fix a stack ouverflow

fis/Managers/Modules/Ultra3D/Ultra3DDownloadWorker.cs

@@ -34,7 +34,6 @@ namespace fis.Win.Dev.Managers.Modules.Ultra3D
 
         public Ultra3DDownloadWorker()
         {
-            _ultra3DStorage = AppManager.Get<IFileStorageManager>().Ultra3DStorage;
            Ultra3DFileWorkItems = new List<Ultr3dUnionWorkItems>();
          
         }
@@ -50,6 +49,10 @@ namespace fis.Win.Dev.Managers.Modules.Ultra3D
                 {
                     continue;
                 }
+                if (_ultra3DStorage == null)
+                {
+                    _ultra3DStorage = AppManager.Get<IFileStorageManager>().Ultra3DStorage;
+                }
                 var surfaceItem = new Ultra3DFileWorkItem(_ultra3DStorage, mainItem.SurfaceUrl, mainItem.DataId);
                 var mdItem = new Ultra3DFileWorkItem(_ultra3DStorage, mainItem.MdFileUrl, mainItem.DataId);
                 var union = new Ultr3dUnionWorkItems(mainItem.DataId, surfaceItem, mdItem);