Guard virtualDesktopManagement being accessed before fetched
The WaylandPrivate::virtualDesktopManagement object is loaded
It's null until we finish fetching the registry.
A code path could call the desktopNames in that time.
Had a crash
Couldn't reproduce reliably
Reviewers: Plasma, hein
Reviewed By: Plasma, hein
Subscribers: hein, plasma-devel
Differential Revision: https://phabricator.kde.org/D19029