The original parent of a dragged actor might have been destroyed after
the drag has been started. When the drag is canceled and _dragOrigParent
is set, the code is trying to get the current position and size of it
when restoring. With a destroyed parent this however would result in a
crash.
This could happen for example when starting a drag on a window preview
while the overview is hiding and then releasing it once the transition
is done.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4024
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1817>