How to Retrieve null datetime in a texbox from gridview ?

Question

i am selection row from gridview in which leftdate is not available/empty ,so when i select that row editing then exception is raised that (String was not recognized as a valid DateTime.')

here is screen shot 

 

here c# code 

 

protected void Edit(object sender, EventArgs e)
        {
            using (GridViewRow row = (GridViewRow)((LinkButton)sender).Parent.Parent)
            {
                txtEmpID.ReadOnly = true;
                txtEmpID.Text = row.Cells[0].Text;
                txtEmpName.Text = row.Cells[1].Text;
                txtFahterName.Text = row.Cells[2].Text;
                //ddlSecID.ClearSelection();
                //ddlSecID.Items.FindByText(row.Cells[3].Text).Selected = true;
                //txtjdate.Text = row.Cells[4].Text;
                txtCNIC.Text = row.Cells[3].Text;
                ddlSecID.ClearSelection();
                if (ddlSecID.Items.FindByText(row.Cells[4].Text) != null)
                {
                    ddlSecID.Items.FindByText(row.Cells[4].Text).Selected = true;
                }
                if (ddlstatus.Items.FindByText(row.Cells[5].Text) != null)
                {
                    ddlstatus.Items.FindByText(row.Cells[5].Text).Selected = true;
                }
                txtjdate.Text = Convert.ToDateTime(row.Cells[6].Text).ToString("dd/MM/yyyy");



                if (!string.IsNullOrEmpty(txtldate.Text))
                {
                    //   DateTime date = DateTime.Parse(txtldate.Text);
                    txtldate.Text = null;
                }
                else
                {
                    txtldate.Text = Convert.ToDateTime(row.Cells[7].Text).ToString("dd/MM/yyyy");
                }

                popup.Show();
            }
        }