Find texbox control inside a gridview in the checkbox event CheckedChanged

protected void chkBoxAplicaSiglasEspEdit_CheckedChanged(object sender, EventArgs e)
    {
        int i;
        int x = gvTipoInforme.Rows.Count;
        CheckBox chkBoxAplicaSiglasEspEdit = (CheckBox)sender;


        TextBox txtSiglas = (TextBox)this.gvTipoInforme.Rows[row].FindControl("txtSiglas");

            for (i = 0; i < x; i++) 
            {
                if (chkBoxAplicaSiglasEspEdit.Checked)
                {
                    txtSiglas.Enabled = true;
                }
                else
                {
                    txtSiglas.Text = "";
                    txtSiglas.Enabled = false;
                }
        }

    }

I'm trying to enable or disable a textbox that is inside a gridview. When I clicked "edit" link a checkbox is shown in a gridview's column, then I click the checkbox and the CheckedChanged event is fired but I'm always getting "txtSiglas" null.