Qt Quick: crash when `delete`-ing a toplevel dockwidget

[MiKom]

Environment

• Qt 6.6.2
• Windows 11
• Visual Studio 2022 compiler 19.38.33135 amd64

Steps to reproduce:

1. Add a timer to examples\qtquick\dockwidgets\main.cpp:

   QTimer::singleShot(std::chrono::seconds(10), [&](){
       delete dw3;
   });

2. Run the application
3. Before the timer runs out, detach the Dock Widget 3 so it forms a separate window
4. Wait until the timer runs out

Expected behavior

The window in which Dock Widget 3 resides, disappears, application keeps running

Actual behavior

Application crashes:

https://github.com/KDAB/KDDockWidgets/assets/104767/3d24dd12-857f-40e9-9b4c-698cae86441a

Stacktrace: https://gist.github.com/MiKom/4d7bb6c25e7b01cc39497f793af96959

[iamsergio]

fixed