Maybe not an issue. just wondering what the best behavior will be aligns people's intuition.
If we simply enter something like
'https://github.com/Siriw3'
we would see a warning saying, 'Please enter a valid username/repository or URL', so maybe using a non-existing repo should throw something similar in the mathwork's generate workflow page as well. However, I do feel that 404 page makes sense as well if you used Github previously.