I don't think it's necessary to redirect to the base URL. At this point, the user can only create or load a project, which will replace the query string anyway. That being said, I can if you really want me to.

I don't think it's necessary to display an error either. It's like if you enter a URL which is a subpage of a website that doesn't exist. Sometimes you get a 404, but other times it just redirects to the main page.