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