Removing node from xml file?

When a user clicks a "Check Out" button, their name is supposed to be removed from an xml file, and subsequently removed from a list box, indicating they left a meeting.  However,I have encountered Null Reference Exceptions during debugging because _xmlNode is null.  From my research on this issue, it seems the problem lies in the declaration of XmlDocument and the passing of its instance _xmlDocument to deleteAttendee().  However, I believe that I declared _xmlDocument correctly, and am having difficulty finding where my strategy went off track.  I would appreciate any pointers that could help steer me back in the right direction.  Thanks!  

protected void checkOutButton_Click(object sender, EventArgs e)
        {
            string fileName = Server.MapPath("~/App_Data/attendees.xml");
            XmlTextReader _xmlTextReader = new XmlTextReader(fileName);
            XmlDocument _xmlDocument = new XmlDocument();
            _xmlDocument.Load(_xmlTextReader);
            _xmlTextReader.Close();

            deleteAttendee(inMeetingListBox.SelectedItem.Text, fileName, _xmlDocument);
            inMeetingListBox.Items.Remove(inMeetingListBox.SelectedItem);


        }
static void deleteAttendee(string attendeeName, string fileName, XmlDocument xmlDoc)
        {
            string test = xmlDoc.DocumentElement.ToString();
            XmlElement _xmlElement = xmlDoc.DocumentElement;
            XmlNode _xmlNode = _xmlElement.SelectSingleNode("attendee[@name='asp']");
            _xmlElement.RemoveChild(_xmlNode);
            xmlDoc.Save(fileName);
        }