Grab first row and first column gridview Javascript

Question

I’m trying to get the value of the first line and the first column of gridview through javascript, but it’s returning error. Keep going like I’m trying to do:

var grid = document.getElementById("#<%=GridView12.ClientID %>");
var cell = grid.rows[parseInt(selectedRowIndex) + 1].cells[0];
var hidID = cell.childNodes[0];
document.getElementById('textcodigo').value = hidID;

Returns me the following error:

Uncaught Typeerror: Cannot read Property 'Rows' of null

I managed to reach this code:

var grid = document.getElementById("<%=GridView12.ClientID %>");
var cell = grid.rows[0].cells[0];
var hidID = cell.childNodes[0];
document.getElementById('textcodigo').value = hidID;

But in textcode appears [Object Text]. I need the first line, the first column.

This is my Gridview12:

<asp:GridView ID="GridView12" runat="server" AutoGenerateColumns="False" DataKeyNames="identificador" CellPadding="4" CssClass="table table-responsive table-hover table-striped" GridLines="None" OnRowDeleting="GridView12_RowDeleting1">
  <Columns>
    <asp:BoundField ItemStyle-Width="100px" DataField="identificador" HeaderText="Identificador" SortExpression="identificador"></asp:BoundField>
    <asp:TemplateField>
      <ItemTemplate>
        <asp:LinkButton CssClass="btn btn-danger btn-sm" ID="deleteButton" runat="server" CommandName="Delete" Text="Excluir" OnClientClick="return confirm('Deseja excluir o identificador selecionado?');"></asp:LinkButton>
      </ItemTemplate>
      <ItemStyle Width="10px" />
    </asp:TemplateField>
  </Columns>
</asp:GridView>

And I arrived in this role, but he keeps returning me Undefined.

var grid = document.getElementById("<%=GridView12.ClientID %>");
var cellPivot;

if (grid.rows.length > 0) {

  var theDropdown = grid.rows[0].cells[0].text;
  document.getElementById('textcodigo').value = theDropdown;

}