Apps SDK docs indicate that the widgetDomain field in resource meta is optional but now i am seeing this message in connection settings indicating that the field is required for submission.
Just noticed this too, it wasn’t there a week ago. I’d strongly assume based on this being a recent change and they’re closing in to going live that yes, yes it is required 
It really is just the FQDN of where you host your app from, which everyone will need I assume. Are you having trouble deciding what it should be?