Sub-windows cannot be displayed normally when the main window is displayed.

当主窗口处于显示状态时，向其中添加一个子窗口部件，然后再次调用显示，子窗口就无法显示了。但如果先隐藏主窗口，然后再显示它，子窗口部件就可以正常显示了

Google translated: When the main window is in the display state, add a sub-window widget to it, then call display again, and the sub-window will not be displayed.