title: Error in event handler for (unknown): TypeError: Cannot read property 'checked' of null
body: <p>the run results is ok,but I don't know why come out this mistake.thanks.
   the browser is Chrome.
   Error:Error in event handler for (unknown): TypeError: Cannot read property 
   'checked' of null. at Object.SL_BBL_locer</p>
  
  <pre><code>&lt;%@ page language="java" contentType="text/html; charset=utf-8"
      pageEncoding="utf-8"%&gt;
      &lt;!DOCTYPE html&gt;
      &lt;html&gt;
      &lt;head&gt;
      &lt;meta http-equiv="Content-Type" content="text/html; charset=utf-8"&gt;
      &lt;title&gt;Insert title here&lt;/title&gt;
      &lt;script type="text/javascript"&gt;
          window.onload=function(){
              window.addEventListener('message',function(e){
                  var color=e.data;
                  console.log(color);
              },false);
              window.frames[0].postMessage('getcolor','http://localhost:8080/HelloWorld/onMessage.jsp');
          }
      &lt;/script&gt;
      &lt;/head&gt; 
      &lt;body&gt;
              &lt;iframe id="child" src="http://localhost:8080/HelloWorld/onMessage.jsp"&gt;&lt;/iframe&gt;
      &lt;/body&gt;
      &lt;/html&gt;
  </code></pre>