Ext.LoadMask在ext6中总是出错

Question

百度出来的教程都是这样写：

 var myMask = new Ext.LoadMask(Ext.getBody(), {msg:"请等候..."});
 myMask.show();

但是这样运行会报错，在源码这里出错：

show: function() {
        var me = this;
        // Element support to be deprecated
        if (me.isElement) {
            me.ownerCt.mask(this.useMsg ? this.msg : '', this.msgCls);
            me.fireEvent('show', this);
            return;
        }
        return me.callParent(arguments);
    }

Cannot read property 'mask' of undefined