cell.parentNode is null error in IE

 if (cell.parentNode.getAttribute("Textbox") =="A")         {

}

prior i was getting no error after adding  below script i get the above error in IE

and that too after the process is over not initally on page load

<script type="text/javascript">
    function beforeAsyncPostBack() {
        var curtime = new Date();
        alert('Time before PostBack:   ' + curtime);
       /

    }

    function afterAsyncPostBack() {
        var curtime = new Date();
        //document.getElementById('Label1').innerHTML = 'Time after PostBack:    ' + curtime;

        alert("after postback");
        
       
        getdetails();
    }

    Sys.Application.add_init(appl_init);

    function appl_init() {
        var pgRegMgr = Sys.WebForms.PageRequestManager.getInstance();
        pgRegMgr.add_beginRequest(BeginHandler);
        pgRegMgr.add_endRequest(EndHandler);
    }

    function BeginHandler() {
        beforeAsyncPostBack();
    }

    function EndHandler() {
        afterAsyncPostBack();
    }
 
       
    </script>