When Deserialize then Nullable type DateTimeOffset? throw exception

public static class Test
    {
        public static testclass Create()
        {
            RefClass2 oRef = new RefClass2()
            {
                Class2ID = ",id2,",
                Class2Name = "name2"
            };
            RefClass2 oRef2 = new RefClass2()
            {
                Class2ID = "id3",
                Class2Name = "name3"
            };
            RefClass1 oTeacher = new RefClass1()
            {
                Class1ID = "id1",
                Class1Name = "name1",
                Class1ObjType = EnumObj1.A,
                RefClass = oRef
            };
            RefClass1 oTeacher2 = new RefClass1()
            {
                Class1ID = "\"id4\"",
                Class1Name = "name4",
                Class1ObjType = EnumObj1.B,
                RefClass = oRef2
            };
            testclass oRet = new testclass()
            {
                Name = "xiao ming",
                Age = 16,
                ShirtColor = ConsoleColor.Blue,
                Mode = StartMode.Manual,
                PreviewExecuteionTime = null,
                NextExecuteionTime = DateTime.Now,
                Teachers = new RefClass1[2] { oTeacher, oTeacher2 }
            };
            return oRet;
        }
    }

    public class testclass
    {
        public string Name { get; set; }

        public int Age { get; set; }

        public string School { get; private set; }

        public ConsoleColor ShirtColor { get; set; }

        public StartMode Mode{get;set;}

        public string ModeText
        {
            get
            {
                if (Mode == StartMode.Auto)
                {
                    return "0";
                }
                return "1";
            }
        }

        public DateTimeOffset? PreviewExecuteionTime { get; set; }

        public DateTimeOffset? NextExecuteionTime { get; set; }

        public RefClass1[] Teachers { get; set; }
    }

    public enum StartMode
    {
        Auto = 0,
        Manual = 1
    }

    public class RefClass1
    {
        public string Class1ID { get; set; }

        public string Class1Name { get; set; }

        public EnumObj1 Class1ObjType { get; set; }

        public RefClass2 RefClass { get; set; }
    }

    public class RefClass2
    {
        public string Class2ID { get; set; }

        public string Class2Name { get; set; }
    }

    public enum EnumObj1
    {
        A = 0,
        B = 1
    }