When using moc in my previews I experienced crashes in preview (canvas). Everything worked on the simulator. The preview would just crash. Looking at the Diagnostics, this was the error reported:

Exception thrown while invoking -[PreviewsInjection.UVPreviewNonUIAgent performUpdate:replyHandler:]: An NSManagedObject of class 'Book' must have a valid NSEntityDescription

The solution proposed in the lesson Showing book details didn't work for me. Solutions to similar problems, like this one from this forum didn't fully resolve the issue either.

Putting the two together solved the issue for me. What I had to do on some preview structures is:

struct ContentView_Previews: PreviewProvider {

    static var dataController = DataController()

    static var previews: some View {
        ContentView()
            .environment(\.managedObjectContext, dataController.container.viewContext)
    }
}

struct DetailView_Previews: PreviewProvider {
    static var dataController = DataController()
    static var moc = dataController.container.viewContext

    static var previews: some View {
        let book = Book(context: moc)
        book.title = "Test book"
        book.author = "Test author"
        book.genre = "Fantasy"
        book.rating = 4
        book.review = "This was a great book; I really enjoyed it."

        return NavigationView {
            DetailView(book: book)
        }
    }
}