Sequence Contains No Elements error when launching Essentials Manager

You receive an error trying to open Essentials Manager - "Sequence contains no elements"

When launching Essentials REST Manager, instead of being presented with a login form or windows prompt, instead we see an error "Sequence contains no elements".

The error message has the styling from the Identity Server login experience. This is caused by having zero hosts in the Hosts... dialog for the service. Running the post installer, clicking the Hosts... button, and then clicking reset will revert the dialog to its original contents.

If the default values do not contain the desired final host name for the server, be sure to add that too..