Issue
After setting up a custom domain, visting the associated .herokudns.com address in my browser shows "There's nothing here, yet."
Resolution
herokudns.com endpoints are not directly browsable and are only to be used as CNAME targets for DNS. The Heroku routing layer uses the Host header from the client to ensure the correct application responds to a request - this needs to be set to the value of your custom domain (if you have one set up) to route to the correct application. As a result, you cannot access your application by visiting the herokudns.com address directly.
If you are using Heroku SSL and you want to check the certificate, you can do this using the openssl command, as described here https://devcenter.heroku.com/articles/ssl#add-certificate-and-intermediaries
For more information using herokudns.com CNAME targets, please see https://help.heroku.com/VKRNVVF5/what-is-the-correct-dns-cname-target-for-my-custom-domains