How to add datestamp to the datagridview

I have an xml file which the user will load to the datagridview and write to the file. all this it working fine. on the xml i have datestamp column which will load the date outomatically and it must also update the time when the user edited from other cell. i tried many examples yesterday but i didnt come with a working solution or from the question i asked. today i created a class to try to update my column but it is not working

when i debug from CellEndEdit and my class throws an error "Object reference not set to an instance of an object." , i cant even step into my class and see where the error it is.

Question : how can i get the time get updated automatically when you edit or write to the file?

my codes

my column for datestamp

       private DataTable GetDataTableFromDataGridview(DataGridView _grid)
    {
      {
             var _oDataTable = new DataTable();
            object[] cellValues = new object[_grid.Columns.Count];
            _oDataTable.Columns.Add("DateStamp", typeof(DateTime));
            foreach (DataGridViewRow row in _grid.Rows)
            {
                for (int i = 0; i < row.Cells.Count; i++)
                {
                    cellValues[i] = row.Cells[i].Value;
                }
                _oDataTable.Rows.Add(cellValues.ToArray());
            }
            return _oDataTable;
        }
    }

     private void Gridview_Output_CellEndEdit(object sender,  DataGridViewCellEventArgs e)
    {
        try
        {
            _UpdateTime.UpdateTimeNow();
        }

}

my class

class UpdateTime { public UpdateTime() { }

    private Regex _dateRegex = new Regex(@"(?<=\[DateStamp\]).*?(?=\[/DateStamp\])");
    public string _DateFormat = "yyyy/dd/MM HH:mm:ss";
    private DateTime _date;
    private XElement _datenodes;

    public string DateStamp
    {
        get
        {
            return _date.ToString(_DateFormat); 
        }
    }

my class

           namespace Project
           {
           class UpdateTime
             {
           public UpdateTime()
            {
              }

    private Regex _dateRegex = new Regex(@"(?<=\[DateStamp\]).*?(?=\[/DateStamp\])");
    public string _DateFormat = "yyyy/dd/MM HH:mm:ss";
    private DateTime _date;
    private XElement _datenodes;

    public string DateStamp
    {
        get
        {
            return _date.ToString(_DateFormat); 
        }
    }

    public void UpdateTimeNow()
    {
        _date = DateTime.Now;
        DateTime _dt;
        this._datenodes.Value = string.Format("[DateStamp]{3}[/DateStamp]",this._date.ToString(_DateFormat, CultureInfo.InvariantCulture));
        if (DateTime.TryParseExact(_dateRegex.Match(this._datenodes.Value).Value, _DateFormat, CultureInfo.InvariantCulture, DateTimeStyles.None, out _dt))
        {
            this._date = _dt;
        }
    }

}

}