Extention becomes unavailable in safari after changes to template

I'm trying to go through the tutorial for creating a safari extension here https://developer.apple.com/documentation/safariservices/safari_app_extensions

When I create a new project from the template it initially runs as expected in safari, but as soon as I make a change to the script.js file the app disappears from the extensions menu in safari preferences. Deleting the changes does not fix the problem. I've tried restarting safari and checking "allow unsigned extensions" but that didn't help either.

I'm getting this error in the console which I don't understand, not sure if it's relevant?

[default] Unable to load Info.plist exceptions (eGPUOverrides)

Any idea what's going wrong?