Object reference not set to an instance of an object

protected void butAdd_Click(object sender, EventArgs e)
   {
       try
       {
           XDocument xmlDoc = XDocument.Load(Server.MapPath("People.xml"));

           xmlDoc.Element("Persons").Add(new XElement("Person", new XElement("Name", txtName.Text),
           new XElement("City", txtCity.Text), new XElement("Age", txtAge.Text)));

           xmlDoc.Save(Server.MapPath("People.xml"));
           jsCall("Data successfully added to XML file.");
           readXML();
       }
       catch
       {
           jsCall("Sorry, unable to process request. Please try again.");
       }
   }