Data Controls :: How To Get Access Elements Inside GridView Using JavaScript

I am trying to get access the elements inside gridview by validating this requirement: if checkbox is checked then 1) Role must be selected from the dropdown 2) And the Title textbook must be filled. I am getting this error now:

Error: Sys.WebForms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object.

JS

<script type="text/javascript">
function validate() {
var flag = true;
var gridView = document.getElementById('<%= gvTest.ClientID %>');
for (var i = 1; i < gridView.rows.length; i++) {

[Code] ......