How Do I Solve Object Reference Not Set To An Instance Of An Object

Am trying to loop through xmlnodes. but getting this error "Object reference not set to an instance of an object." My main goal was to loop it through the datgridviewcell but i don't know how i can take it further. I know the column indexes for font,date and comment from my grid. how can i loop through those column indexes and be able to parse the value to string?

 <?xml version="1.0" encoding="utf-8"?>
 <root>
<data name="Button"xml:space="preserve">
<value></value>
<comment>[Font][/Font][DateStamp][/DateStamp[Comment][/Comment]</comment>
 </data>

     XmlNodeList _Nodelist = _doc.SelectNodes("/root");
            foreach(XmlNode _Xnode in _Nodelist)
            {
                XmlNode _data = _Xnode.SelectSingleNode("data");
                XmlNodeList _CommentNodes = _data.SelectNodes("comment");
                if(_CommentNodes != null)
                {
                    foreach(XmlNode node in _CommentNodes)
                    {
                        XmlNode _comment = node.SelectSingleNode("comment");
                        {

                                string _font = _comment["Font"].InnerText; //it throws the error here
                                string _Date = _comment["DateStamp"].InnerText;
                                string _Comment = _comment["Comment"].InnerText;
                        }
                    }
                }
            }