Object reference not set to an instance of an object.

Object reference not set to an instance of an object. when i try to update my xml document

my xml files is :



 
 
   
      My name is mohammed shamsheer.I am working in poloysys
   
 
 


code behind :

  protected void btnUpdate_Click(object sender, EventArgs e)
    {
        XmlDocument UpdateXmlDocument = new XmlDocument();
        UpdateXmlDocument.Load(Server.MapPath("XMLFile2.xml"));
        XmlNodeList UpdateNodeList;
         UpdateNodeList = UpdateXmlDocument.SelectNodes("/News12/Category[@id='1']");
        UpdateNodeList[0].ChildNodes[2].InnerText = "Asp.net";
       

    }
}