Revi reports four instances (probably one for each file) of this error between the Deed and Details steps of UW. @matmarex says there was a spike in these errors starting on the 17th. Maybe a recently merged patch that has a call to an mw.Title method using undefined instead of a string.
My crystal ball told me that you're selecting the "The license is described by the following wikitext" option on the "Release rights" step (actually, that is the only place where we call mw.Title that has changed recently, so it was an obvious place to look). It seems we don't correctly guard against poorly written scripts polluting global prototypes there, which is what causes this exception.