We've had a look at your repository and the original commit that the pull request was anchored appears to be stripped.
In this situation the pull request is supposed to be auto rejected, but that doesn't appear to have happened. There is also a bug where the update form doesn't work when the anchor commit is broken. I've raised these issue internally and we'll look into it.
In the meantime, the best course of action is to reject the existing pull request and create new one instead.