XML query dataset and to fill a datatable with this dataset

I run this code but I have error on ds.Tables[0].Rows[1][1].ToString()). why?

I can see data in datagridview1.

            string myXMLfile = Application.StartupPath + @"\Query4.xml";
            DataSet ds = new DataSet();
            ds.ReadXml(myXMLfile);
            dataGridView1.DataSource = ds;
            dataGridView1.DataMember = "Query4";
            MessageBox.Show(ds.Tables[0].Rows[1][1].ToString());