When closing a popup (submenu) triggers closing another popup (the menu)
programatically it can happen that QApplicationPrivate::popupWidgets is
destroyed. Therefore we have to check if popupWidgets is still valid
after the focus change event was delivered.
Fixes: QTBUG-81222
Change-Id: Ide3a6897e43f389d396a80d8b158f7c8eb04e3aa
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>