Object reference not set to an instance of an object.

Question

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

my xml files is :

<?xml version="1.0" encoding="utf-8"?>
<News12>
  <Category id="1">
  <News1>
    <News>
      My name is mohammed shamsheer.I am working in poloysys
    </News>
  </News1>
  </Category>
</News12>

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";
       

    }
}