Maybe it's just my setup at work with github enterprise.
I set my github-review-host to "git.mycompany.com" but I needed to use api.git.mycompany.com in order to get things to work.
Otherwise, it bombed out with an unhelpful Got an error from the GitHub API (wrong-type-argument stringp nil)! even though it never sent the request as ghub--auth was erroring out.
Actually, maybe there's a bug somewhere. Because when I call gitreview-submit-commet it uses git.mycompany.com as the host.