It looks like this file is missing from the repo. After running through setup, I get this error when hosting via the Developer Rig:
yarn run v1.7.0
$ node ./scripts/ssl.js && node ./scripts/host.js -l hello-world/public/ -p 8080
SSL Cert already exists at /home/code/git/developer-rig/ssl/selfsigned.crt
Serving assets from https://localhost.rig.twitch.tv:8080
[Sun, 24 Jun 2018 21:30:32 GMT] - "GET /config.html?anchor=config&language=en&mode=config&state=testing&platform=web"
[Sun, 24 Jun 2018 21:30:32 GMT] - "GET /config.js"
[Sun, 24 Jun 2018 21:30:32 GMT]ERROR: "GET /config.js"
{ Error: Not found
at notFound (/home/code/git/developer-rig/node_modules/union/lib/routing-stream.js:88:13)
at dispatch (/home/code/git/developer-rig/node_modules/union/lib/routing-stream.js:105:14)
at Object.onceWrapper (events.js:313:30)
at emitNone (events.js:106:13)
at module.exports.emit (events.js:208:7)
at /home/code/git/developer-rig/node_modules/union/lib/routing-stream.js:114:23
at Object.exports.(anonymous function) (/home/code/git/developer-rig/node_modules/ecstatic/lib/ecstatic/status-handlers.js:37:5)
at fs.stat (/home/code/git/developer-rig/node_modules/ecstatic/lib/ecstatic.js:340:24)
at FSReqWrap.oncomplete (fs.js:152:21) status: 404 }
https://github.com/twitchdev/extensions-hello-world/blob/8b183b213cef0d798e740a045ede704e4f155b87/public/config.html#L13
It looks like this file is missing from the repo. After running through setup, I get this error when hosting via the Developer Rig: