
A boxpointer sourceActor could be destroyed before the boxpointer itself.
In such case, unset the sourceActor reference, connecting to 'destroy' signal.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1295
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/576
(cherry picked from commit 2fd120162f
)