QMainWindow with QGLWidget close loses all child widgets

Closing a QMainWindow using the corner X button causes all child widgets to be lost.

Simple project attached.

• Show the second QMainWindow with a child QGLWidget.
• Close the window programmatically using the custom button.
• Show the window again and see the black QGLWidget.
• Close the window again using the corner X button.
• Show the window again and see a white blank window,

The child widget is lost forever as soon as the native close button is used.