don't assertion fail when user closes window

panda/src/windisplay/winGraphicsWindow.cxx

@@ -727,6 +727,7 @@ window_proc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam) {
       }
     
       case WM_CLOSE:
+        GraphicsWindow::close_window();
         properties.set_open(false);
         system_changed_properties(properties);