D9593: Fix crash in setMainWindow on wayland

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R236 KWidgetsAddons

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D9593

To: fvogt, graesslin, dfaure
Cc: #frameworks