[SNI] Fix race condition in item registration
Summary:
If StatusNotifierItem is registered and then immediately destroyed, it is possible that QDBusServiceWatcher will not emit the serviceUnregistered signal.
Add an additional check to avoid such situations.
BUG: 416652
FIXED-IN: 5.18.0
Test Plan: Telegram should not add empty items.
Reviewers: Plasma: Workspaces, Plasma, davidedmundson
Reviewed By: Plasma: Workspaces, Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27126