Ajax webmethod C# error

[{"ResponseTime":"13/6/2019 24:51","TicketId":0,"UserId":"ricky.global9@gmail.com","Subject":"Domain problem","Department":"Sales","Description":"Content 2","FileId":"NULL","ResponseType":null},{"ResponseTime":"13/6/2019 23:51","TicketId":0,"UserId":"ricky.global9@gmail.com","Subject":"_Payment_issue","Department":"Support","Description":"Content 1","FileId":"NULL","ResponseType":null}]{"d":null}

{"d":null}

<pre>
var tid = '2723021';

$.ajax({
                type: "POST",
                contentType: "application/json; charset=utf-8",
                url: "../../Service.asmx/GetTicketDetails",
                data: "{ TicketID: '" + tid + "'}",
                dataType: "json",
                success: function (data) {
                    var SignalTable = $('#ticketDetails');
                    SignalTable.empty();

                    $(data).each(function (index, sigs) {
                        SignalTable.append('<p>'
                            + sigs.ResponseTime
                            + '</p><p>' + sigs.UserId
                            + '</p><p> ' + sigs.Subject
                            + '</p ><p>' + sigs.Department
                            + '</p><p>' + sigs.Description
                            + '</p><p>' + sigs.FileId + '</p><p>');
                    });
                },
                error: function (err) {
                    alert('Err ' + err);
                }
            });

<pre>
public void GetTicketDetails(string TicketID)
        {
            List<SupportTicket> SideTickets = new List<SupportTicket>();

            string CS = ConfigurationManager.ConnectionStrings["iqCon"].ConnectionString;
            using (SqlConnection con = new SqlConnection(CS))
            {
                SqlCommand cmd = new SqlCommand("procSidebarTicketDetails", con);
//Precedure query SELECT Response_time, User_email, Support_subject, Support_department, Support_description, File_id FROM Support_tickets WHERE Ticket_id = @Ticketid;
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Ticketid", TicketID);

                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();

                while (rdr.Read())
                {
                    SupportTicket tickets = new SupportTicket();
                    tickets.ResponseTime = rdr["Response_time"].ToString();
                    tickets.UserId = rdr["User_email"].ToString();
                    tickets.Subject = rdr["Support_subject"].ToString();
                    tickets.Department = rdr["Support_department"].ToString();
                    tickets.Description = rdr["Support_description"].ToString();
                    tickets.FileId = rdr["File_id"].ToString();
                    SideTickets.Add(tickets);
                }
            }
            JavaScriptSerializer js = new JavaScriptSerializer();
            Context.Response.Write(js.Serialize(SideTickets));
        }

public class SupportTicket
        {
            public string ResponseTime { get; set; }
            public int TicketId { get; set; }
            public string UserId { get; set; }
            public string Subject { get; set; }
            public string Department { get; set; }
            public string Description { get; set; }
            public string FileId { get; set; }
            public string ResponseType { get; set; }            
        }