新手WEBBROWSER提交表单问题请教

我是新手，想用delphi写个程序自动提交表单数据，获取返回表格数据

unit unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, MsHtml, StdCtrls, OleCtrls, SHDocVw, ComCtrls;

type
  TForm1 = class(TForm)
    WebBrowser1: TWebBrowser;
    Edit1: TEdit;
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure WebBrowser1DownloadComplete(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  webbrowser1.Navigate('http://www.');
end;

procedure TForm1.Button2Click(Sender: TObject);
var
begin
    WebBrowser1.oleobject.document.all.item( 'username',0).value := 'aaaaaa';
    WebBrowser1.oleobject.document.all.item( 'password',0).value := '123456';
    WebBrowser1.oleobject.document.all.item('submit').click;
end;


procedure TForm1.FormCreate(Sender: TObject);
begin
  webbrowser1.Navigate('http://www.');
end;

procedure TForm1.WebBrowser1DownloadComplete(Sender: TObject);
begin
  edit1.text:=WebBrowser1.oleobject.Document.all.tags('TABLE').item(7).innertext; //运行此处出错access violation at address 0045ADB3 in module 'project1.exe'
end;

end.

不知道是什么原因，哪位大侠能指点一下，万分感谢！