Uncaught TypeError: Cannot read property 'currentWebview' of undefined

提示错误：
Uncaught TypeError: Cannot read property 'getLength' of undefined at js/pmain/storage.js:10
Uncaught TypeError: Cannot read property 'currentWebview' of undefined at js/mui.min.js:6
代码：
mui.init();
var txtRes=document.getElementById("pdrTxtTestRes");
var stor=null;
var storPlus=null;

mui.plusReady(function(){
stor=window.localStorage;
storPlus=plus.storage;
txtRes.value+="LocalStorage中保存的Key总数为："+stor.length+" \n";
txtRes.value+="PlusStorage中保存的Key总数为："+storPlus.getLength().toString()+" \n";
});