QMainWindow may create unnecessary tab bars for tabified dock widgets if not visible

In a case where a QMainWindow !isVisible and

1. adding new dock widgets
2. tabifying the dock widgets
3. removing and deleting dock widgets

will result in creation of a new child tab bar each time steps 1-3 are repeated.

Steps to reproduce:

1. Run the attached project
2. Click "test" button repeatedly
3. Observe the output - the number of main window children (QTabBars) keeps increasing on each click

Workaround: call activate on the QMainWindow::layout after step 3 or make it visible at least temporarily