[applets/systemtray] Fix invisible plasmoid icons when Media Player is added
Summary:
After adding a new plasmoid (applet), for example a new Media Player, several empty icons are rendered in the system tray which hides existing icons.
The bug occurs after opening Gwenview while the Media Player applet is NOT in use.
The systray icons reappear when Gwenview is closed or another source starts playing.
BUG: 418662
Test Plan: Start Gwenview when Media Player is not in use. All icons should render correctly.
Reviewers: Plasma: Workspaces, Plasma, ngraham, broulik, apol, davidedmundson, mart
Reviewed By: Plasma: Workspaces, Plasma, mart
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D29344