Hybride: SpeedButton, Checkbox and Image

Hello,

I try to make my control change state of other controls of the same
type when they have the same GroupIndex set...
All I get is Access Violation :(

could you please take a look at my code and tell me what is wrong?

Excerpt that makes problems:

________________________________________________________
procedure TxImage.Click;
var i, cc: integer;
begin
  inherited;
  if FGroupIndex <> 0 then
    FChecked := True
  else
    FChecked := not FChecked;

  if Assigned(PictureON) and Assigned(PictureOFF) Then
    if FChecked then Picture.Bitmap := PictureON.Bitmap
      else Picture.Bitmap := PictureOFF.Bitmap;

//THIS PART MAKES PROBLEMS :(((((
  if (FGroupIndex <> 0) and (Parent <> nil) and FChecked then
  begin
    cc := Parent.controlCount -1;
    with Parent do
    begin
      for i := 0 to cc do
       if Parent.Controls[i] is TxImage then
         if TxImage(Parent.Controls[i]).FGroupIndex = FGroupIndex then
           begin
             TxImage(Parent.Controls[i]).Checked := False;
             TxImage(Parent.Controls[i]).Invalidate;
           end;
    end;
  end;
//THIS PART MAKES PROBLEMS :(((((

  invalidate;
end;
________________________________________________________

whole code here:
http://rafb.net/paste/results/qWkjL977.html

Thanks in advance!
~spleen