ModalPopup_Extender.Hide() Issue

ModalPopup_Extender.Hide() Issue

I have a button, when clicked, a modalpopup with gridview inside it. as
below;
protected void grdDetails_SelectedIndexChanged(object sender, EventArgs e)
{
GridViewRow row = grdDetails.SelectedRow;
//Note: Value of that row's cell must display in A text box (txtBox)
on main page
txtBox.Text = row.Cells[2].Text;
ChargeFilterModalDialogExtender.Hide();
}
After the extender is hide, the value of the cell not displaying in the
text box. Am I doing something wrong?