QMenu's causedStack maintains a list of menus on the way to the menu,
and might contain nullptr if one of the entries was a tear-off menu
that got closed (and thus destroyed, due to DeleteOnClose).
If the entry we get from the stack is nullptr, fall back to the passed-
in parent widget pointer, and test for nullptr before accessing.
Add a test case that crashes without the fix.
Fixes: QTBUG-112217
Pick-to: 6.6 6.5 6.2
Change-Id: I958182db47c3cc8733e1780f7efef43881ffae11
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
f0049873d2