NSPersistentContainer未初始化

Question

我将CoreData添加到现有项目中，但我无法将NSPersistentContainer添加到Initialize中。这是我在App Delegate中的代码：

var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: "TimeConverter")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        return container
    }()

只有当我尝试保存managedObjectContext时，应用程序才会崩溃。错误（在App Delegate中）是：

Thread 1: signal SIGABRT

为什么没有初始化？

func saveContext ()
{
    let context = persistentContainer.viewContext
    if context.hasChanges {
        do {
            try context.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nserror = error as NSError
            fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
        }
    }
}

此外，这里是在viewcontroller中调用的地方

 func save(event: String,fromCourse:String,toCourse:String,fromTime:String,convertedTime: String)
{
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate
        else
    {
        return
    }

    let managedContext = appDelegate.persistentContainer.viewContext

    let entity = NSEntityDescription.entity(forEntityName: "ConversionHistory",
                                            in: managedContext)!

    let conversion = NSManagedObject(entity: entity,
                                     insertInto: managedContext);

    conversion.setValue(event,          forKeyPath: "event")
    conversion.setValue(fromCourse,     forKeyPath: "fromCourse")
    conversion.setValue(toCourse,       forKeyPath: "toCourse")
    conversion.setValue(fromTime,       forKeyPath: "fromTime")
    conversion.setValue(convertedTime,  forKeyPath: "toTime")




    do
    {
        try managedContext.save()
      //  people.append(person)
    } catch let error as NSError
    {
        print("Could not save. \(error), \(error.userInfo)")
    }
}