有关DELPHI控制网页菜单栏的问题

有关DELPHI控制网页菜单栏的问题：
运行的时候报地址错误，Access violation at address 0045B6A3,请教大家该怎么解决，或者是换一种方法来处理，我是个新手，谢谢大家。
DELPHI内容:
procedure TForm1.btn1Click(Sender: TObject);
var
doc2:IHTMLDocument2;
o : Olevariant;
ole_index: OleVariant;
frame_dispatch: IDispatch;
frame_win: IHTMLWindow2;
frame_doc: IHTMLDocument2;
begin
doc2 := Wb1.Document as IHTMLDocument2;
ole_index:=0；
frame_dispatch := doc2.Frames.Item(ole_index);
  if frame_dispatch <> nil then
  begin
  frame_win := frame_dispatch as IHTMLWindow2;
  frame_doc := frame_win.document;
  o:=frame_doc.all.item('MainMenu1',0);
  o.onclick; //报地址错
  end;
end;
网页内容摘要：
                <table id="tabMainMenu" border="0" cellpadding="0" cellspacing="0" width="100%" class="nav_mg">
<tr class="nav_m">
<td class="nav_l" id="trMainMenu"></td>
<td class="nav_s" id="FirNav" name="MainMenu0" onclick="ChangeNav(this,'Common/Menu','01','../../PlugIn/ResourceMgt/Resource.htm','','false','')">配置管理</td>
<td class="nav_f" name="MainMenu1" onclick="ChangeNav(this,'Common/Menu','02','../../PlugIn/PerformanceMgt/Performance.htm','','false','')">报表管理</td>
<td class="nav_f" name="MainMenu2" onclick="ChangeNav(this,'Common/Menu','03','../../PlugIn/AlarmMgt/AlarmMgt.htm','','false','')">安全管理</td>
<td class="nav_f" name="MainMenu3" onclick="ChangeNav(this,'Common/Menu','04','../../PlugIn/SystemMgt/SystemMgt.htm','','false','')">系统管理</td>
<td class="nav_f" name="MainMenu4" onclick="ChangeNav(this,'Common/Menu','06','Empty.aspx','','false','')">日志管理</td>
<td class="nav_f" name="MainMenu5" onclick="ChangeNav(this,'Common/Menu','07','Empty.aspx','','false','')">告警查询</td>
<td class="nav_f" name="MainMenu6" onclick="ChangeNav(this,'Common/Menu','08','Empty.aspx','','false','')">通告平台</td>
<td class="nav_f" name="MainMenu7" onclick="ChangeNav(this,'Common/Menu','09','../../PlugIn/About/Help.htm','','false','')">帮助</td>
<td><img src="/WNMSweb/Images/Blue/btn/space.gif" /></td>
<td class="nav_r"></td>
</tr>
</table>