no value in previously cleared databound textboxes (c#)

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C#

combobox

I have a combobox and bound textboxes that show details of a chosen item.
When I choose items, everything works fine. on selected index = 0, new form is opened and if i cancel it, i need the selected index to be set to -1.

So when i do that, the data in the text boxes is left from previously selected item(before index 0 was selected). so i clear the text boxes (textboxes.text = "").

then, when I choose an item from the combobox it selects it but textboxes are empty. If I do another selection then it shows values, so it seems like the clearing txtboxes has something to do with that.

how can I achieve the clearing of the txtboxes after setting index to -1 without preventing no show of data after next selection?

[Edit]
Here is some code:

private void clientsComboBoxEx_SelectedIndexChanged(object sender, EventArgs e)
{
    if (clientsComboBoxEx.SelectedIndex == -1)
    {
        editClientButton.Enabled = false;
        foreach (TextBox tb in groupBox1.Controls.OfType())
        {
            if (tb.Text != "")
            {
                tb.ResetText();
            }
        }
    }
    if (clientsComboBoxEx.SelectedIndex == 0)
    {
        editClientButton.Enabled = false;
        cb = clientsComboBoxEx;
        NewClient newclientform = new NewClient(cb);
        newclientform.ShowDialog();
        clientsComboBoxEx.SelectedIndex = -1;
    }
    if (clientsComboBoxEx.SelectedIndex >0)
    {
        editClientButton.Enabled = true;
    }
}

[/Edit]