"Add button" for adding a city to the "Add a New World Clock" dialog stays greyed out
In gnome-shell 3.30.2 on Fedora Worktation 29, the "Add" button for the world clock dialog of the standard gnome-shell clock applet in the top bar doesn't work. If I want to add a city, and start typing in the dialog, I get suggestions of cities to add, but whatever I try, the "Add" button stays greyed-out. So I can't add any timezone/city to it. The cancel button works fine but the Add button doesn't. It is illustrated in this screenshot: