[Emargency][How to] Checkbox :: Select one at a time

[lighten]

This is the code section from inno setup.My intention is to make two Checkbox where at a time one is being selected.
But this code return error when first checkbox is clicked.

[code] section:

var
Box2,CheckBox: TCheckBox;    //Yes global variable.

procedure CheckBoxOnClick(Sender: TObject);
begin
if Checkbox.Checked then   //generate error
BEGIN
   CheckBox.State := cbUnchecked;
   Box2.State := cbChecked;
END else
BEGIN
   CheckBox.State := cbChecked;
   Box2.State := cbUnchecked;
END;
end;

procedure Box2OnClick(Sender: TObject);
begin
if Box2.Checked then      //generate error
BEGIN
   CheckBox.State := cbChecked;
   Box2.State := cbUnchecked;
END else
BEGIN
   CheckBox.State := cbUnchecked;
   Box2.State := cbChecked;
END;
end;

procedure CreateTheWizardPages;
var
  Page: TWizardPage;
begin
  Page := CreateCustomPage(wpWelcome, 'Support Us', 'If you like this package ,then please support us.');

  CheckBox := TCheckBox.Create(Page);
  CheckBox.Top :=ScaleY(8)+ScaleX(50);
  CheckBox.Width := Page.SurfaceWidth;
  CheckBox.Height := ScaleY(17);
  CheckBox.Caption := 'Do this';
  CheckBox.Checked := True;
  CheckBox.OnClick := @CheckBoxOnClick;
  CheckBox.Parent := Page.Surface;

  Box2 := TCheckBox.Create(Page);
  Box2.Top :=ScaleY(8)+ScaleX(70);
  Box2.Width := Page.SurfaceWidth;
  Box2.Height := ScaleY(17);
  Box2.Caption := 'No,Thanks.';
  Box2.Checked := False;
  Box2.OnClick := @Box2OnClick;
  { }
  Box2.Parent := Page.Surface;
end;



procedure InitializeWizard();
begin
  { Custom wizard pages }
  CreateTheWizardPages;
end;

Returns an error on firstbox click named:

Access violation at address 00444400. Write of address 011ABFF8.
Please tell me where to change..