Fix segfault in key settings menu when worksheet not open
Test Plan:
- Start cantor
- Close backends menu
- Open 'Setting->Key Settings'
- Check, what segfault happends
- Apply patch
- Check, what now all works fine
Reviewers: Cantor, asemke
Reviewed By: Cantor, asemke
Differential Revision: https://phabricator.kde.org/D12354