So you raised your extension frontend on https://localhost:8081/ and your backend on https://localhost.rig.twitch.tv:8081
Probably wise to put them on separate ports.
What did you set your hosting URL to at Extension Dashboard -> Manage Extension -> Versions -> 0.0.1 Manage -> Asset Hosting
Did you put https://localhost:8081 here as you put the later then you are trying to display your EBS/Backend as the extension front end in the frame
Something like:
Make sure to have also set Panel Viewer Path correctly, which based on your screenshots should be viewer.html