Creating a custom page raises an exception

When I attempt to create a new Custom page with an embedded table, I get an error:

plugins.invalid ’s server IP address could not be found.

I’m using the gristlabs/grist:latest Docker image to create the container.

Am I missing a plugin?

Sorry for this bit of ugliness. There is a small bug - custom widget initially shows a welcome message, and that welcome message is served from a domain that needs to be configured separately for completely irrelevant historical reasons. The custom widgets are functional - if you go to the right side bar and put in a url such as that widget will function as described at Custom - Grist Help Center