create_group_model() doesn't handle undefined groups

[Jeremy Whitlock]

create_group_model contains a test for each member of each group, that checks whether the member is really another group. However, get_dict_key_from_value will return None if the member is a group, but that group is not in the dictionary. This can happen if you start your search at a point in the tree below where the group is defined. At this point, create_group_model tries to concatenate None to the string GROUP:, resulting in an exception.

This bit of code needs a trap for get_dict_key_from_value returning None:

key = get_dict_key_from_value(groupmap, mg[0][0][0])
if key == None:
  if verbose:
    print("[WARNING]: this member is a group, but the group is not in the group dictionary:" + mg[0][0][0])
else:
  members.append("GROUP:" + key)