Setting a menu to a location returns an error on /menu-locations

[edtjones]

If you set a menu to a pre-defined menu location, there's an error in the menu-locations endpoint:

Notice: Undefined index: main in /srv/www/rooftop-cms/public/web/app/mu-plugins/wp-api-menus/lib/wp-api-menus-v2.php on line 227

See screenshot too.