Simple app demonstrating how to get a default Core Data app (Xcode 12.4) working with a Picker.
I was struggling to get the selection working with a core data picker and found that the thread below helped, but didn't work in the case that the database was empty.
https://developer.apple.com/forums/thread/674482
So I adapted that code to work with a standard Xcode Core Data template app and this is the result.