I'm going through the bookworm examples, and running into something unexpected: the preview always crashes as soon as you try to use a @FetchRequest variable in your view. Since the same code works fine when running into a simulator, I suspect there is some extra setup that needs to be done in ContentView_Previews ?

It appears to be necessary to do this:

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        let moc = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return ContentView().environment(\.managedObjectContext, moc)
    }
}

That should probably be added to the book?

Even that is not quite good enough: things still break if you try to save anything :(