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