QOpenGLWidget crash when temporary fullscreen

Hi,

When taking a QOpenGLWidget fullscreen by removing its parent and calling `showFullScreen` and then trying to re-attach it, I experience a crash (and the stack indicates it happens in Qt event handling, valgrind does not show any warning).

It seems the event (either keyboard or context menu) is sent to the QWindow that is deleted by re-attaching the QOpenGLWidget to the main window.

I managed to work around the bug by using a Qt::QueuedConnection, but I am afraid this work only because the events have time to get handled and the error may occur again.

Please see the attached reproducer.