How do I access the checkbox in listview?

I'm trying to access the ID as FindControl but it doesn't return the value, giving me the object not instantiated error. How can I access its ID? I've tried several things and so far I don't know how to solve this problem. PS: I'm new to C# programming

ASP.NET

<ItemTemplate >
                                            <tr id="item" runat="server" class="row">
                                                <td class="first">
                                                    <asp:Label ID="lblNumero_titulo" runat="server" Text='<%# Eval("numero_titulo") %>' /><br />
                                                </td>
                                                <td><%# Eval("razao_social") %></td>
                                                <td><%# Eval("hora_entrega")%></td>
                                                <td><%# Eval("quem_recebeu")%></td>
                                                <td>
                                                    <asp:CheckBox ID="testecb" runat="server" /> </td>
                                                <td>
                                                    <asp:Label ID="lblCodEntrega"Visible="false" runat="server" Text='<%# Eval("cod_entrega") %>' /><br />
                                                </td>                                                  
                                            </tr>
                                        </ItemTemplate>

C#

protected void btnPesquisar_Click(object sender, EventArgs e)
    {
        limpaCampos();
        using (PrincipalDataContext pdc = new PrincipalDataContext())
        {

            if (!string.IsNullOrEmpty(txtCodRomaneio.Text))
            {
                var romaneios = pdc.z_stp_anb_transp_receb(Convert.ToInt32(txtCodRomaneio.Text)).ToList();
                var r = romaneios.First();
                var check = lvNotas.FindControl("testecb") as CheckBox;

                if (romaneios != null)
                {

                    if (r.status == 1)
                    {
                       check.Checked = true;
                       check.Enabled = false;

                    }

                    lvNotas.DataSource = romaneios;
                    lvNotas.DataBind();
                }
                else
                {
                    ScriptManager.RegisterStartupScript(this, typeof(Page), UniqueID, "alert('Romaneio não encontrado!')", true);
                }
            }
        }
    }