Code review comment for lp:~salgado/launchpad/bug-462923

Hi Salgado. Great, thank you. Your knowledge of the codebase, and in particular the test harnesses, makes me remember how much I don't know. :-)

As we agreed on IRC, please add something like ``assert self.default_next_url is not None, 'The implementation of %s should provide a value for default_next_url' % (self.__class__.__name__,)`` before line 22 of the diff.

Also, please add a comment here in the MP about why we think it is OK to land this without a concurrent CIP landing.

Thanks again.