I get the same issue in 18.0.3. It works in 17 and 19. Weird I can create a .js file in Chrome and then edit the created one in Firefox but I can’t create a new one. Neither in Chrome nor Firefox can I create a .php file.
I would recommend opening an issue in GitHub. Be sure to post browser you are using plus any errors in the dev tools console.
As a temporary resolution the only thing I can advise is disable and install the old text editor app - following same instructions here: Issue with "Text" App under Internet Explorer 11