Problema con asignar valor nil a un puntero

[kdemia]

El error es :

Debugger Exception Notification
---------------------------
Project Project1.exe raised exception class EAccessViolation with message 'Access violation at address 004B5657 in module 'Project1.exe'. Write of address 00000008'.

Igualmente el metodo create esta demas, era para ver si el problema se encontraba en la asignacion esa.. pero no tendria porque pasar, osea si hago una cola de manera procedural funciona perfectamente, el problema esta en hacerlo en poo.

Igualmente tmb dejo el codigo del Form principal.

Código Delphi [-]
unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  Cola : TCola;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  Cola.Push(Edit1.Text);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  ShowMessage(Cola.Pop);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Cola.Create;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  Cola.Free;
end;

end.