[MM3-users] Deleting default "example.com" domain breaks everything

Hello. I'm new with v3 of mailman. Installed via aptitude on deb10. After I logged in as admin, I created my actual domain, as "example.com" was there by default. I then deleted example.com, and everything broke. The only site I had was still using example.com. How do I either a) restore example.com in config or CLI (as web UI is completely broken now), or b) move my one and only "Site 1" to the domain I created?

Thanks.