Thanks for this, but com.apple.dt.Xcode doesn't seem to exist on my system

I have reinstalled Xcode and the simulator files. What files are you referring to?

Solved: there was a private key missing

I have tried building documentation for the SlothCreator package using Xcode 13 beta and Mac OS Monterey beta and it doesn't open the documentation window (as per the demo). It just opens the Developer Documentation window. I guess that the developers have a later version of Xcode to be released rsn.