Adding columns doesn't seem to work in the developer center: https://streamable.com/mrerb . A friend of mine tried it as well, but he faced the same issue. What's wrong here?

 

Edit: When I rightclick on "Add new column" -> open link in new tab it will work just fine. So probably there is something wrong with your popup window / javascript?

Edit 2: Is there a reason that you don't support the unique modifier for columns?

I can't reproduce. It seems like you're facing a javascript error, so you may want to check your browser console for details.