When deleting the panel the notifications applet is in, the containment is...

When deleting the panel the notifications applet is in, the containment is being destroyed but the corona
is still there and rightfully emits availableScreenRectChanged.
This will cause us to crash when we try to access the no-longer existing containment.

CCBUG: 378508

Differential Revision: https://phabricator.kde.org/D5321