I can reproduce the strange blank dialog, but creating the channel works for me when I say “Yes.”
A workaround that seems to work for now is to create the channel without “Automatically add users” selected, and then go back and edit the channel to turn that on.
Thanks Dave, I was able to create a second channel in that manner… however when I try to create the one I originally wanted to it does not work (clicking on creating channel does nothing) and whenever I click on create channel I get this in console: