how to free main form when MDI form close

I write a MDI form (not child) as :
procedure TForm1.Button1Click(Sender: TObject);
begin
 Minitool := TMinitool.Create(Self);
 Hide;
 Application.NormalizeTopMosts;
 SetWindowPos(Application.Handle, HWND_TOPMOST, 0,0,0,0,
      SWP_NOACTIVATE+SWP_NOMOVE+SWP_NOSIZE);
 Minitool.ShowModal;
end;

procedure TMinitool.FormDestroy(Sender: TObject);
begin
 Application.Terminate ;
end;

I want the MDI form always on top, it does well, but when it destroy the
main form (form1) cannot release, what wrong did i make ?