[applets/systemtray] Fix closing active applet by clicking it again

This was removed by mistake in https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/285


(cherry picked from commit bd2fcb05b1)

applets/systemtray/package/contents/ui/items/PlasmoidItem.qml

@@ -87,6 +87,7 @@ AbstractItem {
             } else if (root.activeApplet === applet) {
                 // if not expanded we don't have an active applet anymore
                 root.activeApplet = null;
+                dialog.visible = false;
             }
         }