QWidget destructor always deletes all its children, which is fatal
in case not-yet-deleted frame (e.g. because it's in lazy delete queue)
points to one of those children

There's parent-child relationships between the frames though, so perhaps
no need to have it double. For native modality this seems enough

Change-Id: I556ace8d7949bcdfb208170cc08181ac1f2622e8
Reviewed-on: https://gerrit.libreoffice.org/57560
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>