swift エラーメッセージ

###前提・実現したいこと
課題期限通知アプリを作りたいです

###発生している問題・エラーメッセージ

Could not cast value of type '__NSDate' (0x104c92080) to 'NSString' (0x10429dc60).```

viewDidLoadでエラーが起こっているようです

###該当のソースコード

swift
1import UIKit
2
3class FirstViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
4    
5    @IBOutlet var tableView: UITableView!
6    var todoarray = [String]()
7    var todoarraydate = [String]()
8    var refresher: UIRefreshControl?
9    
10    override func viewDidLoad() {
11        super.viewDidLoad()
12        if UserDefaults.standard.array(forKey: "todoarray") == nil {
13            todoarray = [String]()
14            UserDefaults.standard.set(todoarray, forKey: "todoarray")
15        } else {
16            todoarray = UserDefaults.standard.array(forKey: "todoarray") as! [String]
17        }
18        
19        if UserDefaults.standard.array(forKey: "todoarraydate") == nil {
20            todoarraydate = [String]()
21            UserDefaults.standard.set(todoarraydate, forKey: "todoarraydate")
22        } else {
23            todoarraydate = UserDefaults.standard.array(forKey: "todoarraydate") as! [String]
24        }
25        
26        refresher = UIRefreshControl()
27        
28        refresher?.attributedTitle = NSAttributedString(string: "Pull to refresh")
29        
30        refresher?.addTarget(self, action: #selector(FirstViewController.refresh), for: UIControlEvents.valueChanged)
31        
32        self.tableView.addSubview(refresher!)
33        
34        refresh()
35        // Do any additional setup after loading the view, typically from a nib.
36    }
37    
38    override func didReceiveMemoryWarning() {
39        super.didReceiveMemoryWarning()
40        // Dispose of any resources that can be recreated.
41    }
42    
43    internal func numberOfSections(in tableView: UITableView) -> Int {
44        return 1
45    }
46    
47    func refresh() {
48        tableView.reloadData()
49        refresher?.endRefreshing()
50    }
51    
52    internal func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
53        if todoarray.count == todoarraydate.count {
54            return todoarray.count
55        } else {
56            return todoarraydate.count
57        }
58    }
59    
60    internal func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
61        var cell = tableView.dequeueReusableCell(withIdentifier: "cell")
62        if cell == nil {
63            cell = UITableViewCell(style: .default, reuseIdentifier: "cell")
64        }
65        cell?.textLabel?.text = todoarray[indexPath.row]
66        cell?.detailTextLabel?.text = String(describing: todoarraydate[indexPath.row])
67        return cell!
68    }
69    
70    internal func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {
71        // Return false if you do not want the specified item to be editable.
72        return true
73    }
74    
75    @IBAction func new(_ sender: AnyObject) {
76        self.performSegue(withIdentifier: "New To Do", sender: self)
77    }
78    
79    func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {
80        if editingStyle == .delete {
81            todoarray.remove(at: indexPath.row)
82            todoarraydate.remove(at: indexPath.row)
83            tableView.deleteRows(at: [indexPath], with: .fade)
84            UserDefaults.standard.set(todoarray, forKey: "todoarray")
85            UserDefaults.standard.set(todoarraydate, forKey: "todoarraydate")
86        }
87    }
88    
89    //画面遷移
90    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
91        
92        performSegue(withIdentifier: "toSubViewController", sender: nil)
93    }
94    
95}