silva-1.2 kupu bug with lists and headings

Hi!

We've just uncovered a bug in silva-1.2.1 + kupu-1.2.1.  Here's how to 
reproduce:

create a new document.  In kupu, type some text, turn it into a list (ordered 
or unordered, doesn't matter), and change the text's style to 'sub'.  If you 
try to save, you'll get a javascript alert: 'response status 500'.  The error 
log has this traceback:

Traceback (innermost last):
  Module ZPublisher.Publish, line 101, in publish
  Module ZPublisher.mapply, line 88, in mapply
  Module ZPublisher.Publish, line 39, in call_object
  Module Products.SilvaDocument.Document, line 258, in PUT
  Module Products.SilvaDocument.Document, line 169, in editor_storage
  Module UserList, line 28, in __getitem__
IndexError: list index out of range

Before I create an issue for this, I wanted to see if someone could check 
Silva-1.3 to see if it has this problem too?  I don't have the beta 
installed.

Cheers,
Andy