Getting QWindow back into parent QWidget from the fullscreen crashes

I have an object that subclasses QWindow. I add it into my QMainWindow's layout by wrapping it into QWidget container like this:

{{myWinContainer= QWidget::createWindowContainer(myWindow,this); vbox->addWidget(myWinContainer); myWinContainer->setFixedSize(640,360); }}

Then there is a fullscreen mode. I tried to set screen mode directly on myWindow (QWindow):

{{myWindow->setParent(nullptr); myWindow->showFullScreen(); }}

The windows goes full screen all right. But I can't get it back into its widget when exiting from the full screen.

{{myWindow->setWindowState(Qt::WindowNoState); myWindow->setParent(p);//where p is cached pointer to parent: myWindow->parent() myWindow->showNormal(); }}

The window stays detached from the main window.

Then I tried to do the full screen via myWindow's parent widget (myWinContainer). In this case, every time I am inserting myWinContainer back into its parent :

{{vbox->insertWidget(0,myWinContainer); }}

layout object, it crashes with  internal error:

Program: D:\Qt\5.9.1\msvc2015_64\bin\Qt5Cored.dll
Module: 5.9.1
File: global\qglobal.cpp
Line: 3049
ASSERT: "toplevel->windowHandle()" in file kernel\qwindowcontainer.cpp, line 380

Other people from the  Qt dev forum who tried this, reported the same issue. Here is the link to the thread:

https://forum.qt.io/topic/82879/getting-qwindow-back-into-parent-qwidget-from-the-fullscreen/3