When no central widget has been set, calling takeCentralWidget should
just return a null pointer instead of crashing.
[ChangeLog][QtWidgets][QMainWindow] Fixed crash using takeCentralWidget when
the central widget was not set.
Task-number: QTBUG-56628
Change-Id: I240ccf4caa41d2716a78851571fbfbf444a4922e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>