SwiftUI Preview has error "A fetch request must have an entity"

Change

struct SheetMusicView_Previews: PreviewProvider {
    static var previews: some View {        
        SheetMusicView()
            .environment(\.managedObjectContext, PersistenceController.shared.container.viewContext)
    }
}

To

struct SheetMusicView_Previews: PreviewProvider {
    static var previews: some View {
        let context = PersistenceController.shared.container.viewContext
        return SheetMusicView()
            .environment(\.managedObjectContext, context)
    }
}