Some progress - remove_face_uv still crashes

Discovered it was writing empty vertex lists. That was because tessfaces were not generated before being read and thus the list was empty.
Now, when tessfaces are generated, the problem seems to be that remove_face_uv is not working correctly because not all triangles have face_uv correctly initialized.
It would help immensenly to actually know what is the purpose of remove_face_uv, actually it is not even clear to me what an UV is, or why it is necessary in the file. Go figure