[applets/SystemTray] Fix crash on shutdown

Summary:
When having expanded System Tray once, plasmashell crashes on teardown.
Fixes regression introduced in D27085.

BUG: 417552
FIXED-IN: 5.19.0

Test Plan:
1. Start plasmashell
2. Open System Tray popup, close it again
3. run kquitapp5 plasmashell, it should not crash

Reviewers: #plasma_workspaces, #plasma, broulik, ngraham

Reviewed By: ngraham

Subscribers: anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27401