Guard virtualDesktopManagement being accessed before fetched

Summary:
The WaylandPrivate::virtualDesktopManagement object is loaded
asyncronously.

It's null until we finish fetching the registry.

A code path could call the desktopNames in that time.

Test Plan:
Had a crash
Couldn't reproduce reliably

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: hein, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19029