What version are you running?
ReviewBoard 5.0 (just upgrade from 3.0.15
What's the URL of the page containing the problem?
/r/new/
What steps will reproduce the problem?

type anything to the filter on the left.
choose any repository.
upload a diff file.


What is the expected output? What do you see instead?
Expected output: move to review request draft
Actual output: stuck with loading icon.
And server response with 404 (trying to access /nullapi/validation/diffs/)

What operating system are you using? What browser?
Ubuntu 20.04 and chrome 106.0.5249.119, but it seems irrelevent.
Please provide any additional information below.
It seems repository from filtered result will have localSitePrefex = null, instead of "".
https://github.com/reviewboard/reviewboard/search?q=this.get%28%27localSitePrefix%27%29
You can see the validateDiffModel.es6.js handle the attribute diffently from the others. so the null is being prepend to the api url cause the problem.