Two things here:
- fix ghash lookup from rB4d497721ecd1
-- this was looking in the wrong map (causing an assert on file load)
- set MovieTrackingObject active_plane_track to NULL upon deletion (same
as for regular tracks)
-- rBfe38715600c introduced a crash because `draw_tracking_tracks` would
still get an active plane track (logic for getting these changed)
Maniphest Tasks: T102887
Differential Revision: https://developer.blender.org/D16660
9f3b0e41bb