¿Cómo crear (cuando quiero mostrarlo) y destruir (cuando quiero ocultarlo) cuadros en el TForm principal? Frames 'align = alClient.

Intenté esto:

La forma:

unit main;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls, uFrame1, uFrame2;

type
  TFormMain = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    f1: TFrame1;
    f2: TFrame2;
  end;

var
  FormMain: TFormMain;

implementation

{$R *.dfm}

procedure TFormMain.FormCreate(Sender: TObject);
begin
  f1 := TFrame1.Create(Self);
  f1.Parent := Self;
end;

end.

Primer cuadro:

unit uFrame1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;

type
  TFrame1 = class(TFrame)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

implementation

{$R *.dfm}

uses main, uFrame2;

procedure TFrame1.btn1Click(Sender: TObject);
begin
  Self.Free;
  FormMain.f2 := TFrame2.Create(FormMain);
  FormMain.f2.Parent := FormMain;
end;

end.

Segundo cuadro:

unit uFrame2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;

type
  TFrame2 = class(TFrame)
    lbl1: TLabel;
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

implementation

{$R *.dfm}

uses main, uFrame1;

procedure TFrame2.btn1Click(Sender: TObject);
begin
  Self.Free;
  FormMain.f1 := TFrame1.Create(FormMain);
  FormMain.f1.Parent := FormMain;
end;

end.

pero se bloquea con las rutas de acceso cuando hago clic en el botón FrameStart o Frame1 (TForm FormCreate funciona bien, es decir, crea y muestra FrameStart).

Delphi 7.