ดึงรูป(Binary)ใน Database มาแสดง บน Gridview ไม่ขึ้นครับมันบอก Object reference not set to an instance of an object.

SqlDataAdapter da = default(SqlDataAdapter);
            DataTable dt = new DataTable();
            SQL = "select * from TestPic  where picName= @picName";      
            da = new SqlDataAdapter(SQL, conn);           
            //cmd = da.SelectCommand();
            SqlCommand cmd = new SqlCommand(SQL, conn);
            
            cmd.Parameters.Add("@picName", SqlDbType.Int).Value = Request.QueryString["picName"].ToString();                
            da.Fill(dt);
            

            if (dt.Rows.Count > 0)
            {
                Response.ContentType = dt.Rows[0]["picType"].ToString();
                Response.BinaryWrite((byte[])dt.Rows[0]["picName"]);
            }