Add null-pointer checks to QCocoaEventDispatcher:: currentModalSession(): - window->handle() may return nullptr if the window has been destroyed. - We call beginModalSessionForWindow, which processes events. This can potentially destroy or delete the current window; pointers to it must be checked again. This also has the effect that currentModalSessionCached is not set to a session that does not have a window, also prevents clearing the cleanupModalSessionsNeeded flag for such sessions. Task-number: QTBUG-66536 Change-Id: Ie055933c872a8ede3c938882fb2d4f7cf8ff3c81 Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>