swiftui crashes not working with coreData

Question

I'm supporting iOS 15, my code is working properly at runtime, but my previews are crashing. I understand that maybe the issue is about providing them a managed context , but really do not understand how to do it. I found someone found solutions, but not working for me. I'm not using AppDelegate. regarding error, sometime I get "an nsmanagedobject of class lesson must have a valid nsentitydescription" sometimes the preview simply crashes with no message and a black screen with a red "x"

import SwiftUI

@main
struct CoreDataIOS15FilesAddedApp: App {
    
    @StateObject private var dataController = DataController()
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, dataController.container.viewContext)
        }
    }
}


import CoreData
import Foundation
class DataController: ObservableObject {
    
    let container = NSPersistentContainer(name: "Bookshelf")
    
    init() {
        
        container.loadPersistentStores { description, error in
            if let error  {
                print("error: \(error.localizedDescription)")
            } else {
                print("✅ loaded persistent stores")
            }
        }
    }
    
}


import SwiftUI

struct ContentView: View {
    
    @Environment(\.managedObjectContext) var moc
    @FetchRequest (sortDescriptors: [], predicate: nil) var lessons: FetchedResults<Lesson>
    
    @State var showAddView = false
    
    var body: some View {
        NavigationView {
            VStack {
                List {
                    ForEach(lessons, id: \.self) { lesson in
                        NavigationLink {
                            DetailView(lesson: lesson)
                        } label: {
                            HStack {
                                Text( "\(lesson.mainAssertion ?? "ND")")
                                    .font(.body)
                            }
                            
                        }
                    }
                }
                .sheet(isPresented: $showAddView) {
                    AddDataView()
                }
                //************************************************************
                .toolbar {
                    ToolbarItem(placement: .navigationBarLeading) {
                        Button {
                            showAddView = true
                        } label: {
                            Label("Add Lesson", systemImage: "plus")
                        }
                    }
                    
                }
            }
            .padding()
        }
        
    }
    
}

//#Preview {
//    ContentView() //crashes when I save
//}

import SwiftUI

struct AddDataView: View {
    
    @Environment(\.managedObjectContext) var moc
    @Environment(\.dismiss) var dismiss
    
    @State private var mainAssertion: String = ""
    @State private var origin: String = ""
    
    var body: some View {
        NavigationView {
            VStack {
                
                Form {
                    TextField("MainAssertion", text: $mainAssertion)
                    TextField("Origin", text: $origin)
                }
                
                Button("Add") {
                    let newLesson = Lesson(context: moc)
                    
                    newLesson.mainAssertion = mainAssertion
                    newLesson.origin = origin
                    
                    do {
                        try  moc.save()
                        dismiss()
                    } catch {
                        print(error.localizedDescription)
                    }
                }
            }
            .navigationBarTitle("Add Data", displayMode: .inline)
            .navigationBarItems(trailing: Button("Cancel") {
                dismiss()
            })
        }
    }
}


//#Preview {
//    AddDataView() //crashes when I save
//}



import SwiftUI

struct DetailView: View {
    
    let lesson: Lesson
    
    @Environment(\.managedObjectContext) var moc
    @Environment(\.dismiss) var dismiss
    
    @State private var showingDeleteAlert = false
    @State private var genericAlert = false
    
    @State private var alertTitle = ""
    @State private var alertMessage = ""
    
    var body: some View {
        Form {
            Section {
                DeatailRowView(name: "MainAssertion", info: "\(lesson.mainAssertion ?? "ND" )")
                DeatailRowView(name: "Origin", info: "\(lesson.origin ?? "ND")")
            }
        }
        .navigationTitle("Detail View")
    }
}

//struct DetailView_Previews: PreviewProvider {
//    static var previews: some View {
//        DetailView(lesson: Lesson())
//    }
//}