[away3d] Prefab: how to make a model with null material

[Ralph B]

I am trying to use Prefab to convert an OBJ file to an AS3 file with
null material, because I want to apply a ShadingColorMaterial to the
whole object later at the mesh level.

I have a simple test example (TestHouse.obj in the group files area).
If I import it into the latest Prefab then export to Away3D AS3, it
creates an "images" subfolder and places a new jpg there.  Inside the
AS3 I can see that it is applying that jpg as a bitmap.

How should I use Prefab to output the AS3 without any bitmap material?

Thanks!
Ralph