"Names of friends" cannot be empty

Dear support,

today I wanted to try one option in settings, I wanted to know how it works - Geocaching - Names of friends. I put some friend's name there, checked what it does, I did not like it, so I wanted to delete that friend from that list - to leave the option empty as it was before. And this is where my problem is - when I delete the name and hit "Set" button, I get message "Field cannot be empty". Actually, "empty" is exactly what I need in order to disable this setting. Can you please fix this issue so I could remove the friend from the list?

Thanks in advance.

Martin