Using 
QWidget.createWindowContainer on Qt3DExtras.Qt3DWindow(), and placing it in a layout, along with other widgets that are willing to take space, causes the app to crash.

In this example code, a horizontal box layout is used, the widgified Qt3DExtras.Qt3DWindow() is added first to the layout, then a sub-layout holding sliders is added, if the sliders in horizontal orientation, it will cause the app to crash, if they are in vertical orientation, the app works fine.

To fix it, I have to manually set a minimum or fixed width for the widgified Qt3DExtras.Qt3DWindow() to force it to take space, making the app work find all the time.