Crash on edge case while grouping frameless dock widgets while floating.

Usually I make dock widgets frameless and use a custom title bar widget on them, this is to enforce the app style on them, now the crash happens if I enabled GroupDragging dock option in the QMainWindow, it works correctly until an edge case happens.

I included a minimal example and a video showing the bug in detail, and here's the steps to reproduce the bug using the example:-

1-Drag one of the dock widgets outside the app to make it float.

2-Drag other dock widget on the latter one to group them, this will show a tabbar.

3-Now inside the new group you have just made, drag one of the dock widgets from the tabbar, outside the new group, and the magic will happen.

4-Either the app will crash now or a sudden empty glitch window will appear, if you happen to drag any of the dock widgets to that glitch window, they will refuse, but if you happen to drag a group of dock widgets on it, the app will crash.

5-If you happen to close the glitch window, already grouping is permanently bugged, spawning glitch windows all over the place when any two dock widgets try to group up.