Onappear引起了预览的问题，但没有显示错误

Question

自我学习初学者。

当我删除.onappear {add（）}时，预览效果很好。我试图将其连接到其他视图，即VSTACK，但会导致另一个错误。我阅读/观看了几个教程，但没有提到任何内容。...

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struct ListView: View {

@Environment(\.managedObjectContext) var moc
@FetchRequest(sortDescriptors: []) var targets: FetchedResults<TargetEntity>
@FetchRequest(sortDescriptors: []) var positives: FetchedResults<PositiveEntity>

var body: some View {
    
    VStack {
        Text("+")
        .onAppear{add()} 
        .onTapGesture (count: 2){
            do {
                increment(targets.first!) //I also sense that doing "!" is not good.  But it's the only way I can keep it from causing error "Cannot convert value of type 'FetchedResults' to expected argument type 'X'"
                try moc.save()
            } catch {
                print("error")
            }
        }
    }
}
    
func increment(_ item: TargetEntity) {
        item.countnum += 1
        save()
}

func add() {
    let countnum = TargetEntity(context: moc)
    countnum.countnum = 0
    save()
}

func save() {
    do { try moc.save() } catch { print(error) }
}

}

编辑20220509： 正如@YRB的建议（非常感谢），错误可能是由于缺乏持久文件中预览VAR的适当设置而引起的。我在此处发布相关代码以进行Visiblity。

数据控制器文件

import CoreData
import Foundation

class DataController: ObservableObject {
    let container = NSPersistentContainer(name: "CounterLateApr")
    
    init () {
        container.loadPersistentStores { description, error in
            if let error = error {
                print("Core Data failed to load: \(error.localizedDescription)")
            }
        }
    }
}

预览代码在视图

struct ListView_Previews: PreviewProvider {
    static var previews: some View {
        NavigationView{
            ListView()
        }        
    }
}

[appName] .app文件中

import SwiftUI

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