What you’re looking at is the last request that a browser makes just as the user redirects away from your app. It’s a last-ditch attempt to fire an event on the session.
Where is it that you are seeing the 404 error occur?
FWIW, the browser would show a 404 error if the app goes away before the browser has a chance to disconnect. I see you’re running locally, so if this is a debug run, it’d be when you stop debugging. In that case, it’s safe to ignore.