Lazy guard for KServiceGroup::root() being null.
Summary:
This will happen if KSycoca::ensureCacheValid fails. (which shouldn't
ever happen).
Hopefully it'll just call refresh later. In any case, we won't
crash.
BUG: 363339
Test Plan: None.
Reviewers: Plasma, mart
Reviewed By: mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D2341