单webview下拉刷新，提示mui.ajax not a function问题

使用hello mui 单webview下拉刷新遇到初始化页面进入查询没问题，下拉刷新后，调用mui.ajax获取新数据，提示找不到mui.ajax

var _self;  
            if(window.plus) {  
                plusReady();  
            } else {  
                document.addEventListener("plusready", function() {  
                    plusReady();  
                }, false);  
            }  

            function plusReady() {  
                _self = plus.webview.currentWebview();  
                _self.setPullToRefresh({  
                    support: true,  
                    height: '50px',  
                    range: '100px',  
                    style: 'circle',  
                    offset: '1px'  
                }, pulldownRefresh);  

                plus.key.addEventListener("backbutton",function () {  
                    _self.close("auto");  
                },false);  
            }  

            function addData() {//alert(pageNum+"----------"+pageSize+"---------"+user.GUID);     
                ajax_message({  
                    page:pageNum,  
                    pageSize:pageSize,  
                    userId:user.GUID  
                });  
            }  
            /**  
             * 下拉刷新具体业务实现  
             */  
            function pulldownRefresh() {  
                pageNum=1;  
                setTimeout(function() {  
                    addData();  
                    _self.endPullToRefresh();  
                }, 1500);  
            }  

            addData();  
            mui = {};  
            mui.back = function () {  
                _self.close("auto");  
            }