Domain Mapping Plugin cannot delete domain bug

I've been having a problem with this plugin, I just updated to the most recent version and I still can't delete a custom domain that has previously been mapped to a site. The site has one custom domain. In Tools > Domain Mapping I click on the delete button for the custom domain and nothing happens.

This custom domain can no longer be used on this site and so the site now cannot be accessed because it tries to redirect to the custom domain which is no longer available. How do I fix this?