Workaround crash on lockscreen close

Summary:
See QTBUG-55460 for details. It's causing our unit test crash when
closing the lock screen with SIGTERM.

It's not worth fixing "properly" as it goes away with QtQuickControls2

Test Plan:
gdb showed it was this crash, which I'd seen before

Ran locker --test
sent SIGTERM
locker closed, but didn't crash

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8100