The draft PR is here:
https://github.com/NixOS/nixpkgs/pull/104888
Basically the error I get is:
Which I don’t know how to solve. The settings window works and seems functional (thanks to a patch I applied) but I can’t enable the extension at all.
Yea I was aware of that, and the patch that fixed it, but I forgot to take the patch correctly (fixed outputs derivations 
): Allow extension to load under GNOME 3.38 by Ian2020 · Pull Request #276 · EasyScreenCast/EasyScreenCast · GitHub