I hit this on try earlier today with Google. I was having some internet blips though, so I assumed it was that. Will see if I can repro.
Yep, consistent repro on try (can’t test here on Meta due to 2FA). I hit https://try.discourse.org/auth/google_oauth2/null after clicking “Log in”, then “Google”, then selecting my account. Login is successful, loading https://try.discourse.org/ directly after hitting the null page works.