Tabbar-Iframe-GetElementByID and make it disappear problem

I need to access and modify a specific element contained in an iframe inside a tab.
I have tried a lot of ways but because I am new in javascript and dhtmlx programming I can’t figure out a way to do this.

The thing that I want to do is to remove a specific div (with the id=“border-top”) from a page that it loads inside an iframe that a tabbar contains!

**The page that I want to modify is mine on a localhost server so I have the right to modify it.

Here is the code I’m using now.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

	<title>DHTMLX Package</title>
	<link rel="STYLESHEET" type="text/css" href="../JoomlaAdminPanel/dhtmlxTabbar/codebase/dhtmlxtabbar.css">
	<script  src="../Example/dhtmlxTabbar/codebase/dhtmlxcommon.js"></script>
    <script  src="../Example/dhtmlxTabbar/codebase/dhtmlxtabbar.js"></script>
    <script src="../Example/dhtmlxTabbar/codebase/dhtmlxcontainer.js" type="text/javascript"></script>

<div id="a_tabbar" style="width:100%; height:100%; z-index: 1;">

tabbar = new dhtmlXTabBar("a_tabbar", "top");
tabbar.addTab("a1", "Example", "100px");
tabbar.setContentHref("a1", "http://localhost/Example/Example1/index.php?option=com_modules");


var ifr;
ifr = tabbar.cells("a1")._frame;
ifr.getElementById("border-top").style.display = 'none';




Try the next:

ifr = tabbar.cells("a1").getElementsByTagName("iframe")[0]; doc = ifr.contentDocument; doc.getElementById("border-top").style.display = 'none';

First of all thank you very much for replying on my topic.
Secondly I used the code you gave me but I get this error ( On chrome Inspect Element>Console):

Uncaught TypeError: Cannot read property ‘style’ of null

Attach you a code sample
Sample.rar (1.51 KB)