Not sure if this helps, but I just figured out that if you don't do modelContext.insert before you access its nested model property (like the Author? in your case), the app will crash with a SwiftData access error (SwiftData error code is "0x1cb459e90") even you wrap the code block with a do-try-catch.
Maples7
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Got the same issue here. The funniest thing is that I have a very similar model with the error one works pretty well. It looks like SwiftData for now is really buggy and error-prone...