我发布了这个问题：
SwiftUI: deleting Managed Object from cell view crashes app?
当我试图了解它崩溃的原因时，我尝试将模型Item更改为timestamp非可选：

extension Item {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Item> {
        return NSFetchRequest<Item>(entityName: "Item")
    }

    @NSManaged public var timestamp: Date

}

extension Item : Identifiable {

}

正如Asperi所指出的，使用这个：

if let timestamp = item.timestamp {
    Text(timestamp, formatter: itemFormatter)
  }

修复了timestamp为可选时的崩溃。
然而，这只是我为了理解如何正确构建视图而测试的一些代码，我需要使用没有可选属性的模型，因此我不能求助于上面链接到的问题的答案。
因此，这个问题是为了解决CellView在ManagedObject上使用非可选属性的场景。
如果我不使用CellView，直接把这段代码放到ContentView中，它不会崩溃。

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        Text(item.timestamp, formatter: itemFormatter)
                    } label: {
//                        CellView(item: item)
                        HStack {
                            Text(item.timestamp, formatter: itemFormatter) // <<- CRASH ON DELETE
                            Button {
                                withAnimation {
                                    viewContext.delete(item)
                                    try? viewContext.save()
                                }
                                
                            } label: {
                                Text("DELETE")
                                    .foregroundColor(.red)
                            }
                            .buttonStyle(.borderless)
                        }
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }

    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.forEach { item in
                viewContext.delete(item)
            }

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

然而，我需要知道如何保留CellView，使用@ObservedObject并使其工作。在这种情况下，这样做并不是什么大不了的事情，但在CellView大得多的真实的情况下，这种方法不能很好地扩展。无论如何，为什么在单独的视图中使用@ObservedObject是错误的呢？
那么，当timestamp在模型中不是可选的时候，为什么应用程序会崩溃呢？为什么视图会试图为一个被删除的项目重绘CellView呢？如何解决这个问题呢？
为了清楚起见，我在这里发布了非可选情况下的新代码，因此您不必返回并查看链接的问题，然后将其更改为非可选。这是整个崩溃的代码：

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        Text(item.timestamp, formatter: itemFormatter)
                    } label: {
                        CellView(item: item)
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }

    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.forEach { item in
                viewContext.delete(item)
            }

            do {
                try viewContext.save()
            } catch {
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

struct CellView: View {
    
    @Environment(\.managedObjectContext) private var viewContext
    @ObservedObject var item:Item
    
    var body: some View {
        
        HStack {
            
            Text(item.timestamp, formatter: itemFormatter) // <<- CRASH ON DELETE
            
            Button {
                withAnimation {
                    viewContext.delete(item)
                    try? viewContext.save()
                }
                
            } label: {
                Text("DELETE")
                    .foregroundColor(.red)
            }
            .buttonStyle(.borderless)
        }
        
    }
    
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}