handle NS_DESTROY message in tree window event handler.

webshell/tests/viewer/nsBrowserWindow.cpp

@@ -760,6 +760,11 @@ HandleTreeWindowEvent(nsGUIEvent *aEvent)
 	  NS_IF_RELEASE(pEnumerator);
 	  NS_IF_RELEASE(pTree);
   }
+  else if ( aEvent->message == NS_DESTROY ) {
+	NS_IF_RELEASE(aEvent->widget);
+	result = nsEventStatus_eConsumeDoDefault;
+  }
+  
   return result;
 }