Fix crash due to dangling reference

Summary:
Returning a reference to a value on the stack is broken.
This caused kwin_wayland to crash in openQA with a nullptr deref.

BUG: 407199

Test Plan: Only build tested.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21085