mui.plusReady内代码不执行

<div id="sheet1" class="mui-popover mui-popover-bottom mui-popover-action ">  
    <!-- 可选择菜单 -->  
    <ul class="mui-table-view">  
      <li class="mui-table-view-cell">  
        <a href="#">1菜单1</a>  
      </li>  
      <li class="mui-table-view-cell">  
        <a href="#">2菜单2</a>  
      </li>  
    </ul>  
    <!-- 取消菜单 -->  
    <ul class="mui-table-view">  
      <li class="mui-table-view-cell">  
        <a href="#sheet1"><b>3取消</b></a>  
      </li>  
    </ul>  
</div>  
<script type="text/javascript" charset="utf-8">  
    mui.init();  
    mui.plusReady(function(){  
        alert("ddd");  
        mui(".mui-content").on("tap","#dd",function(event){  
        mui('#sheet1').popover('toggle');  
        });  

var xhr = new plus.net.XMLHttpRequest();

xhr=new plus.net.XMLHttpRequest();
xhr.onreadystatechange=function(){
console.log("onreadystatechange: "+xhr.readyState);
}
xhr.open( "POST", "" );
var data={key:'ct2'};
xhr.setRequestHeader('Content-Type','application/json');
// 发送HTTP请求
xhr.responseType = "json"
xhr.onload = function() {
console.log(xhr.response);
}
xhr.send(JSON.stringify(data));

    });  
</script>