オプショナルのエラー

前提・実現したいこと

ListのデータをUserdefaultsで保存しようと考えています。
以前の質問の回答を参考にして、DecodableとCodableを使用しています。
以下のエラーメッセージの解決方法がわかりません。

発生している問題・エラーメッセージ

 Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value

該当のソースコード

Swift
1
2import SwiftUI
3
4struct Operate: Identifiable, Encodable, Decodable{
5    
6    var id = UUID()
7    var task: String
8    var time: String
9    
10    
11}
12
13extension Encodable {
14
15    var json: Data? {
16        let encoder = JSONEncoder()
17        return try? encoder.encode(self)
18    }
19}
20
21extension Decodable {
22
23    static func decode(json data: Data?) -> Self? {
24        guard let data = data else { return nil }
25        let decoder = JSONDecoder()
26        return try? decoder.decode(Self.self, from: data)
27    }
28}
29
30extension UserDefaults {
31
32    func set(_ value: Codable?, forKey key: String) {
33        guard let json: Any = value?.json else { return } // 2020.02.23 追記参照
34        self.set(json, forKey: key)
35        synchronize()
36    }
37
38    func codable<T: Codable>(forKey key: String) -> T? {
39        let data = self.data(forKey: key)
40        let object = T.decode(json: data)
41        return object
42    }
43}
44
45var ListWidth = UIScreen.main.bounds.width
46var ListHeight = UIScreen.main.bounds.height
47
48struct OperateList: View {
49    
50    @State var pretask: String = ""
51    @State var pretime: String = ""
52    @State var operate: [Operate] = [Operate(task: "task", time: "60")]
53    
54    var body: some View {
55        
56        VStack{
57            
58            HStack{
59                
60                
61                TextField("task", text: self.$pretask).textFieldStyle(RoundedBorderTextFieldStyle()).frame(width: ListWidth / 2, height:  ListHeight / 20)
62                //taskの入力
63                
64                TextField("time", text: self.$pretime).textFieldStyle(RoundedBorderTextFieldStyle()).frame(width: ListWidth / 5, height: ListHeight / 20).keyboardType(.numberPad)
65                //timeの入力
66                
67                Button(action: {
68                    self.operate.append(Operate(task: self.pretask, time: self.pretime))
69                    self.pretask = ""
70                    self.pretime = ""
71                    UserDefaults.standard.set(self.operate, forKey: "operate2")
72                }){
73                    
74                    ZStack{
75                        RoundedRectangle(cornerRadius: 5).frame(width: ListWidth / 7, height: ListHeight / 20).foregroundColor(.green)
76                        Text("追加").foregroundColor(.white)
77                    }
78                    
79                }
80                //taskとtimeの追加
81                
82            }
83
84            
85            
86            
87            NavigationView {
88                List{
89                    
90                    ForEach(operate, id: .id){num in
91                        
92                        HStack{
93                            Text(num.task)
94                            Spacer()
95                            Text("(num.time)min")
96                        }
97                        
98                                
99                            
100                    }
101                    .onDelete(perform: delete)
102                    .onMove(perform: move)
103                    
104                    
105                }
106                .navigationBarItems(trailing: EditButton())
107                .navigationBarTitle(Text("Operate"), displayMode: .large)
108                
109            }
110            
111            
112        }.onAppear(){
113            
114            let restoreList: [Operate]? = UserDefaults.standard.codable(forKey: "operate2")
115            self.operate = restoreList!　**ここにエラーが出ます**
116            
117        }
118
119        
120        
121        
122     
123    }
124    
125    
126    
127    func delete(at offsets: IndexSet){
128        operate.remove(atOffsets: offsets)
129    }
130    
131    func move(from source: IndexSet, to destination: Int){
132        operate.move(fromOffsets: source, toOffset: destination)
133    }
134    
135        
136}
137
138
139
140struct ToDoList_Previews: PreviewProvider {
141    static var previews: some View {
142        OperateList()
143    }
144    
145}
146