That was an easy one.

Change this line:

  self.tpolys = [tgrab(p, tsets) for p in tpolys]

into this:

  if tsets:
    self.tpolys = [tgrab(p, tsets) for p in tpolys]
  else:
    self.tpolys = []

-- I'm not mad at you, just Westphalian.