There is a cornercase when the user edits an uvmap, that is not part of
the material (yet). When this is the case the uvmap was not added to the
uv buffer and the 'pos' alias was not created.
This change will always request the active uv map when uv editing.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D6534
4aca02064f