If you open Settings → Asterisk SIP Settings, and don’t change anything, and click Submit … it gives an error that “External Address cannot be blank if SIP Legacy is set to Static IP” … even though Legacy SIP isn’t being used at all.
This happens even on a fresh install of v15.
So, the default settings are invalid (from the UI perspective).
If you want to leave the External Address blank, you must go to the Legacy SIP tab and put something (anything) in the static IP field, or change Legacy SIP to Public.
The biggest issue is that if you don’t realize this, you will end up putting something into the External Address blank to get around the error, but in doing so, you will be enabling NAT on PJSIP and you will break your external connections.
I propose that, by default, Legacy SIP should be set to Public so that there is no error message when you click Submit.