String was not recognized as a valid DateTime.'

Question

When i selecting row from gridview for editng and does not has date in date column then this exception is raising 

String was not recognized as a valid DateTime.'

 

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;

                ddlSecID.ClearSelection();
                if (ddlSecID.Items.FindByText(row.Cells[3].Text) != null)
                {
                    ddlSecID.Items.FindByText(row.Cells[3].Text).Selected = true;
                }
                txtjdate.Text = Convert.ToDateTime(row.Cells[4].Text).ToString("dd/MM/yyyy");

                txtldate.Text = Convert.ToDateTime(row.Cells[5].Text).ToString("dd/MM/yyyy");

                popup.Show();
            }