Fix segfault in key settings menu when worksheet not open

Test Plan:

1. Start cantor
2. Close backends menu
3. Open 'Setting->Key Settings'
4. Check, what segfault happends
5. Apply patch
6. Check, what now all works fine

Reviewers: Cantor, asemke

Reviewed By: Cantor, asemke

Subscribers: Cantor, KDE Edu

Tags: KDE Edu, Cantor

Differential Revision: https://phabricator.kde.org/D12354