How to get access elements inside GridView using JavaScript in ASP.Net

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++) {
            var inputs = gridView.rows[i].getElementsByTagName('input');
            var dropdowns = gridView.rows[i].getElementsByTagName('Select');
            var areas = gridView.rows[i].getElementsByTagName('text');

            if (inputs != null && inputs.length >= 1 && inputs[0] != null && areas != null && areas.length > 1 && areas[0] != null && dropdowns != null && dropdowns.length >= 1 && dropdowns[0] != null) {

              if (dropdowns[0].type == "select-one" && inputs[0].type == "checkbox" && areas[0].type == "text") {
                var ddlSelectedItem = dropdowns[0].value;
                var txtval = areas[0].value;

                if (inputs[0].checked && ddlSelectedItem == "Select" && (txtval == "" || txtval == null)) {

                    flag = false;
                    break;
                  }
                  else {
                    flag = true;
                  }
                }
              }
            }

          if (!flag) {
            dropdowns[0].focus();
            alert('Please note that if checkbox is checked then a Role and Title must be selected.  Thanks');      
        }
        return flag;
      }
      </script>

aspx

<asp:GridView ID="gvTest" runat="server"
            PageSize="100">
            <Columns>
            
              <asp:BoundField DataField="Task" HeaderText="Title" HtmlEncode="true" />
                      
               <asp:TemplateField HeaderText="Role">
              <ItemTemplate>
                <asp:DropDownList ID="ddlRole" runat="server">
                </asp:DropDownList>
              </ItemTemplate>  
            </asp:TemplateField>

              <asp:TemplateField HeaderText="Task">
              <ItemTemplate>
                <asp:TextBox ID="txtTsk" runat="server"></asp:TextBox>           
              </ItemTemplate>            
            </asp:TemplateField>  

              <asp:TemplateField HeaderText="Check">              
                  <ItemTemplate>
                    <asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="true"  />
                  </ItemTemplate>
                </asp:TemplateField>        
            </Columns>           
          </asp:GridView>


                <asp:LinkButton ID="ButtonAssign" runat="server" Font-Size="Small"  OnClick="btnAssign_Click" CausesValidation="true" OnClientClick="return validate();"  > 
                       <i aria-hidden="true" class="glyphicon glyphicon-ok"></i>Assign Role
                </asp:LinkButton>

thanks