Select รูปภาพ Parameter แต่ Object reference not set to an instance of an object.

 constring = "server= OHM-PC\\OHM; user id= sa;password=123456;Database=NewTest2;Max Pool Size=400;Connect Timeout=600;";
            conn.ConnectionString = constring;
            conn.Open();
            
            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"]);
            }
            
            conn.Close();
            conn = null;
        }

            SQL = "select * from TestPic  where picName= @picName"; 
            SqlCommand cmd = new SqlCommand(query, conn);
            cmd.Parameters.AddWithValue("@picName", Request.QueryString["picName"].ToString());
            SqlDataReader dr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
            DataTable dt = new DataTable();
            dt.Load(dr);

            if (dt.Rows.Count > 0)
            .............................