Web UI fails to load - can't find assets

After starting up a concourse web worker, I’m unable to load the page in a browser. I’m seeing this error printed in the log each time I attempt to load the page:

2020/06/29 21:06:25 http: panic serving 127.0.0.1:54586: template: index:8:63: executing “index” at <asset “main.css”>: error calling asset: file does not exist

Full message: https://pastebin.com/eKBmsY9C
Configuration: Concourse 6.3.0 running on macOS.

Where do I get the assets from? Thanks.