IFRAME元素访问给空

Question

的HTML是这样

<iframe id="MainFrame"> 
    <div id="Maintest">This is main text 
     <div id ="SecondTest">This is second layer 
      <div id="ThirdTest">This is third layer</div> 
     </div> 
    </div> 
</iframe> 

的JavaScript来存取元件中的帧是这样

var iframe = document.getElementById('MainFrame'); 
    var innerDoc = iframe.contentDocument || iframe.contentWindow.document; 
    console.log(iframe); 
    var mydiv = innerDoc.getElementById("Maintest"); 
    console.log(mydiv); 
    var seconddiv = innerDoc.getElementById("SecondTest"); 
    var thirddiv = innerDoc.getElementById("ThirdTest"); 
    console.log(seconddiv+thirddiv); 

mydiv，seconddiv以及thirddiv给null.I在许多帖子上看到，这是访问iframe元素的方式。我不知道为什么它给null.Help赞赏。

感谢 斯瓦拉杰