Fix "PanelSpacer::containmentGraphicObject()" plasmashell segfault

This one-liner fix is from Aleix Pol, and fixes the following:

- Crashes when deleting panels with Panel Spacer instances on them
  (as described in the bug ID named above)
- Potentially endless crash looping when running `plasmashell --replace`
  while a Panel Spacer is on at least one panel

Co-authored-by: Aleix Pol <aleixpol@kde.org>

BUG: 450663
FIXED-IN: 5.25.3


(cherry picked from commit 65fe6b864d)

applets/panelspacer/plugin/panelspacer.cpp

@@ -122,6 +122,7 @@ PlasmaQuick::AppletQuickItem *PanelSpacer::twinSpacer() const
 
 PlasmaQuick::AppletQuickItem *PanelSpacer::containmentGraphicObject() const
 {
+    if (!containment()) return nullptr; // Return nothing if there is no containment to prevent a Segmentation Fault
     return containment()->property("_plasma_graphicObject").value<PlasmaQuick::AppletQuickItem *>();
 }