On plasmashell shutdown when the panels are destroyed, the available screen geometry changes.
When an applet then accesses screenGeometry in response to that change, it crashes because the corona is already gone.

availableScreenRegion() and availableScreenRect() already checked for this but screenGeometry() did not

No longer have plasma crash in shutdown because of notification popup placement

Ideally, plasmashell didn't bother signalling those changes on shutdown