无法在Swift 3上显示表格视图

Question

我目前正在尝试使用这些代码加载我的tableview，但是在返回 listitem.count 时我得到一个线程断点。任何帮助解决这个问题都会很棒。

以下是我的代码：

import UIKit
import CoreData

class ViewInventoryTableController: UITableViewController{

    var listItems = [NSManagedObject]()

    override func viewDidLoad(){
        super.viewDidLoad()
        //Do any additional setup after loading the view, typically from a nib.
        self.navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.add, target: self, action: #selector(ViewInventoryTableController.addItem))

    }

    func addItem(){
        let alertMessage = "Please key in the activation code"
        let alertController = UIAlertController(title: "Add Item", message: alertMessage as String, preferredStyle: .alert)

        //Add the textfield
        var activationCodeTextField1: UITextField?
        alertController.addTextField { (textField) -> Void in

            activationCodeTextField1 = textField
            activationCodeTextField1?.placeholder = "123456789"
        }

        // Create Cancel button
        let cancelAction = UIAlertAction(title: "Cancel", style: .cancel) { (action:UIAlertAction!) in
            print("Cancel button tapped")
        }
        alertController.addAction(cancelAction)

        // Create ConfirmAction
        let confirmAction = UIAlertAction(title: "Confirm", style: UIAlertActionStyle.default, handler: ({
            (_) in
            if let field = alertController.textFields![0] as? UITextField{

                self.saveItem(itemToSave: field.text!)
                self.tableView.reloadData()

            }
        }
                                                                                                         ))
        alertController.addAction(confirmAction)

        // Present Dialog message
        OperationQueue.main.addOperation {
            self.present(alertController, animated: true){}
        }
        print("Activation Code = \(activationCodeTextField1?.text)")

    }
    //function save item
    func saveItem(itemToSave: String){
        let  appDelegate = UIApplication.shared.delegate as! AppDelegate

        let managedContext = appDelegate.persistentContainer.viewContext


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

        let item = NSManagedObject(entity: entity!, insertInto: managedContext)

        item.setValue(itemToSave, forKey: "item")
        do {
            try managedContext.save()

            listItems.append(item)
        }
        catch {
            print("error")
        }
    }

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

        let  appDelegate = UIApplication.shared.delegate as! AppDelegate

        let managedContext = appDelegate.persistentContainer.viewContext
        tableView.reloadRows(at: [indexPath], with: UITableViewRowAnimation.right)

        managedContext.delete(listItems[indexPath.row])
        listItems.remove(at: indexPath.row)
        self.tableView.reloadData()

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        //Dispose of any resources that can be recreated
    }

    override func tableView (_ tableView: UITableView , numberOfRowsInSection section: Int) -> Int {
        return listItems.count
    }
    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "Cell")! as UITableViewCell

        let item = listItems[indexPath.row]

        cell.textLabel?.text = item.value(forKey: "item") as? String

        return cell
    }

}