This seems like a paradox to me. When Edna prevented Doc from seeing Frankenstein, it created a timeline where Doc never invents the time machine. This means that all of the time traveling adventures should have been erased, including all of Marty's interactions with 1931. But we see in Doc's office that there is a picture of old Doc and Marty in 1931 still.
How is this possible? The problem is that this whole timeline without the time machine can only exist as a result of the time machine. Technically, the universe should have imploded at this point. :confused: