Photo Library access is only supported with Catalina (macOS 10.15) or later. That’s because Catalina is the first OS that allows 3rd party apps to read/write the Photo library.
I’m puzzled that you got as far as you did with the Photo library. There should be blocks in the app for Photo Library on Mojave (I just checked my Mojave install and the New Photo Library window is disabled). So, there must be a bug in the app that is letting that through – so, sorry for that.
The Photo extension, however, works fine on Mojave because that was added in a much earlier release of MacOS (Sierra or maybe El Capitan – I don’t recall).