解决此对象引用未设置为对象实例的问题

基本上，我只是在写一些代码来更新配置文件。但我得到了这个错误，它说"对象引用没有设置为对象的实例"。但我已经试着找到这个错误两天了，但我仍然坚持这个错误。请帮我解决这个错误：(谢谢…

输出：

查看输出

Asp.net.cs代码：

public partial class EditProfile : System.Web.UI.Page
    {
        SqlConnection conn = null;
        SqlCommand cmd = null;
        string connectionString = null;
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                SqlDataReader dr = null;
                connectionString = ConfigurationManager.ConnectionStrings["LeaveManagementCS"].ConnectionString;
                conn = new SqlConnection(connectionString);
                string sql = "SELECT * FROM Staff";
                string id = Session["StaffId"].ToString();
                Session["StaffId"] = id;
                try
                {
                    cmd = new SqlCommand(sql, conn);
                    conn.Open();
                    dr = cmd.ExecuteReader();
                    dr.Read();
                    id = dr["StaffId"].ToString();
                    tbStaffName.Text = dr["StaffName"].ToString();
                    tbPassword.Text = dr["Password"].ToString();
                    tbEmail.Text = dr["Email"].ToString();
                    tbPhoneNo.Text = dr["PhoneNo"].ToString();
                    ddlTitle.SelectedItem.Text = dr["Title"].ToString();
                    dr.Close();
                }
                catch (Exception ex)
                {
                    lblOutput.Text = "Error Message:" + ex.Message;
                }
                finally
                {
                    if (conn != null)
                        conn.Close();
                }
            }
        }
        protected void btnUpdate_Click(object sender, EventArgs e)
        {
            connectionString = ConfigurationManager.ConnectionStrings["LeaveManagementCS"].ConnectionString;
            conn = new SqlConnection(connectionString);
            string sql = "UPDATE Staff SET Username=@username, Password=@Pwd, StaffName=@staff, Email=@email, PhoneNo=@phone, Title=@title ";
            sql += " WHERE StaffId=@id";
            string title= ddlTitle.SelectedItem.Text;
            string id = Session["StaffId"].ToString();
            Session["StaffId"] = id;
            string username = Session["Username"].ToString();
            Session["Username"] = username;
            try
            {
                cmd = new SqlCommand(sql, conn);
                if(username != null)
                {
                    cmd.Parameters.AddWithValue("@username", username);
                }
                cmd.Parameters.AddWithValue("@id", id);
                cmd.Parameters.AddWithValue("@staff", tbStaffName.Text);
                cmd.Parameters.AddWithValue("@Pwd", tbPassword.Text);
                cmd.Parameters.AddWithValue("@email", tbEmail.Text);
                cmd.Parameters.AddWithValue("@phone", tbPhoneNo.Text);
                cmd.Parameters.AddWithValue("@title", title);
                conn.Open();
                int rows = cmd.ExecuteNonQuery();
                if (rows > 0)
                {
                    lblOutput.Text = "Record update successfully";
                }
            }
            catch (Exception ex)
            {
                lblOutput.Text = "Error Message: " + ex.Message;
            }
            finally
            {
                if (conn != null)
                    conn.Close();
            }
            try
            {
                SqlDSEditProfile.Update();
                lblOutput.Text = "Application update";
            }
            catch (Exception ex)
            {
                lblOutput.Text = ex.Message;
            }
        }
    }

            string id = String.Empty;
            if (Session["StaffId"] != null)
            {
                id = Session["StaffId"].ToString();
            }
            Session["StaffId"] = id;
            string username = String.Empty;
            if (Session["Username"] != null)
            {
                username = Session["Username"].ToString();
            }
            Session["Username"] = username;