Not sure why you are getting null.
You can only create a FaceWall from a face that belongs to a Mass or Element of GenericModel category (not a column). As in the UI Face walls are typically created from the results of massing studies i.e. a skin to the face of the mass.
You should check:
FaceWall.IsValidFaceReferenceForFaceWall
FaceWall.IsWallTypeValidForFaceWall