plus.downloader 无法监听？

逻辑是 首页 ajax远程获取是否有新版本 然后下载

    if(e.index == 0){  
                                        if(isAndroid){  
                                            var wgtWaiting = plus.nativeUI.showWaiting("开始下载");  
                                            var dtask = plus.downloader.createDownload(data.url, {}, function (d, status){  
                                                if(status == 200){  
                                                    wgtWaiting.setTitle("开始安装");  
                                                    plus.runtime.install(d.filename);  
                                                }else{  
                                                    mui.alert('下载失败，请稍候再试！');  
                                                    wgtWaiting.close();  
                                                }  
                                            });  
                                            /*dtask.addEventListener("statechanged", function (download, status){  
                                                switch (download.state){  
                                                    case 2:  
                                                        wgtWaiting.setTitle("已连接到服务器");  
                                                        break;  
                                                    case 3:  
                                                        var percent = download.downloadedSize / download.totalSize * 100;  
                                                        wgtWaiting.setTitle("已下载 " + parseInt(percent) + "%");  
                                                        break;  
                                                    case 4:  
                                                        wgtWaiting.setTitle("下载完成");  
                                                        break;  
                                                }  
                                            }, false);*/  
                                            dtask.start();    

控制台提示 TypeError: Cannot read property 'start' of null

如果不监听直接.start 则一切正常