We might run an event loop after destroying qApp and additional events
may be generated from that. Therefore we should not access qApp
unchecked from event handlers. In most places we did already check qApp
for null. Now we do it everywhere.

Task-number: QTBUG-60000
Change-Id: Id72bbf133989e4f8127d85ee63cba64f915d3649
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>