[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

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

@@ -37,10 +37,6 @@ AbstractItem {
     status: applet ? applet.status : PlasmaCore.Types.UnknownStatus
     active: root.activeApplet !== applet
 
-    Component.onDestruction: {
-        applet = null
-    }
-
     onClicked: {
         if (applet && mouse.button === Qt.LeftButton) {
             applet.expanded = true;
@@ -65,7 +61,7 @@ AbstractItem {
 
     //This is to make preloading effective, minimizes the scene changes
     function preloadFullRepresentationItem(fullRepresentationItem) {
-        if (fullRepresentationItem && applet.fullRepresentationItem.parent === null) {
+        if (fullRepresentationItem && fullRepresentationItem.parent === null) {
             fullRepresentationItem.width = expandedRepresentation.width
             fullRepresentationItem.width = expandedRepresentation.height
             fullRepresentationItem.parent = preloadedStorage;