[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:
- Start plasmashell
- Open System Tray popup, close it again
- 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