QPlaceHolderItem is created when removing a dock widget from QMainWindow

See also QTBUG-72266

Using save-/restoreState operation in some cases I was wondering why the saved byte stream growth. This happens, when playing with dockables that are created dynamically during runtime and the number varry from time to time.

The reason for the enlargement is that the QPlaceHolderItems are outputted even for deleted dock widgets.

I tried to find a way to avoid the growth but from the QMainWindow api I can only call removeDockWidget. Unfortunately this operation creates a QPlaceHolderItem and thus the entry of that QDockAreaLayoutItem remains into the QDockAreaLayoutInfo. I did not find any way to prevent this.

Please help.