Error on column with autocomplete

try
{
    
    string headerText = dgvAddKontoNr.Columns["Konto"].HeaderText;

    if (dgvAddKontoNr.CurrentCell.ColumnIndex == dgvAddKontoNr.Columns["Konto"].Index)
    {
        if (headerText.Equals("Konto"))
        {
            if (e.Control is TextBox)
            {
                tb = e.Control as TextBox;
                if (tb != null)
                {
                    tb.AutoCompleteMode = AutoCompleteMode.Suggest;
                    tb.AutoCompleteSource = AutoCompleteSource.CustomSource;
                    AutoCompleteStringCollection coll = new AutoCompleteStringCollection();
                    tb.AutoCompleteCustomSource = coll;
                    con.Open();
                    SqlDataAdapter da = new SqlDataAdapter("SELECT CAST(Kontonummer AS nvarchar (255)) + ' - ' + Navn AS Kontonummer FROM Kontoplan WHERE LockKonto = 'True' AND KlientId = (SELECT Id FROM Klient WHERE Navn = '" + bogf.cmbKlient.Text + "')", con);
                    DataTable dt = new DataTable();
                    da.Fill(dt);

                    for (int i = 0; i < dt.Rows.Count; i++)
                    {
                        string konto = dt.Rows[i]["Kontonummer"].ToString();
                        //string navn = dt.Rows[i]["Navn"].ToString();
                        coll.Add(konto);
                        //coll.Add(navn);
                    }
                    con.Close();


                }
                tb.MinimumSize = new Size(345, 25);
                tb.BorderStyle = BorderStyle.Fixed3D;
            }
            
        }

        else
        {
            TextBox tb = e.Control as TextBox;

            if (tb != null)
            {
                tb.AutoCompleteMode = AutoCompleteMode.None;
            }
        }
    }    
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}