Hello Stefan.
You probably skipped a step.
The error says that you are trying to save a record to the PersonMovieRole, but you are not providing the MovieId value.
The exercise, item 6, is the item that tells how to create this:

If you did this, and this assign is BEFORE the CreatePersonMovieRole

And you are still experiencing the problem, the error possibly is in the previous page (the MovieDetail), as you need to pass the MovieId when navigating to this page, that is made in the 1.g (Add participants to a movie)

Check if you are not passing NullIdentifier().
I am delivering a class right now and the documentation seems in order here.
Cheers.