extjs4.1 desktop 桌面窗口切换，放大缩小，关闭，多操作几次，窗口常会卡死在那， 错误代码：Uncaught TypeError: Cannot read property ‘dom’ of null  或 Uncaught TypeError: Cannot read property ‘dom’ of undefined   定位到 desktop.js文件的 last.el.dom 这行代码：

  updateActiveWindow: function () {
        var me = this, activeWindow = me.getActiveWindow(), last = me.lastActiveWindow;
        if (activeWindow === last) {
            return;
        }

        if (last) {
            if (last.el.dom) {
                last.addCls(me.inactiveWindowCls);
                last.removeCls(me.activeWindowCls);
            }
            last.active = false;
        }

解决办法： 判断 last.el 是否为空 如果为空，没有对象，则中止窗口继续操作

    updateActiveWindow: function () {
        var me = this, activeWindow = me.getActiveWindow(), last = me.lastActiveWindow;
        if (activeWindow === last) {
            return;
        }

        if (last) {
            if (Ext.isEmpty(last.el)) { return; }; //增加这一行
            if (last.el.dom) {
                last.addCls(me.inactiveWindowCls);
                last.removeCls(me.activeWindowCls);
            }
            last.active = false;
        }

增加一行代码 if (Ext.isEmpty(last.el)) { return; };  问题解决，大大减少卡死现象。