shutdown old thread

QuickLook/ViewerWindow.Actions.cs

@@ -273,6 +273,7 @@ namespace QuickLook
             //Dispatcher.BeginInvoke(new Action(Hide), DispatcherPriority.ApplicationIdle);
 
             ViewWindowManager.GetInstance().ForgetCurrentWindow();
+            BeginClose();
 
             ProcessHelper.PerformAggressiveGC();
         }
@@ -280,6 +281,7 @@ namespace QuickLook
         internal void BeginClose()
         {
             UnloadPlugin();
+            busyDecorator.Dispose();
 
             Close();