Avoid crash when there is no QScreen attached to the QWindow

Since QWindow::screen() can return 0 we need to account for this instead
of assuming that it will always be valid.

Task-number: QTBUG-65857
Change-Id: Ifb3a9fdecc76c5c930282101bf383802bf2f2464
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>