问题：
在使用Ext JS Grid的分组和行插件实现四层Grid ，在最下层点击收合时出现如下错误：
完整的错误日志是：
Grouping.js?_dc=1685692706926:923 Uncaught TypeError: Cannot read property 'isCollapsed' of undefined
at constructor.isExpanded (Grouping.js?_dc=1685692706926:923)
at constructor.onGroupClick (Grouping.js?_dc=1685692706926:1174)
at constructor.fire (Event.js?_dc=1685692706927:483)
at constructor.doFireEvent (Observable.js?_dc=1685692706926:816)
at constructor.doFireEvent (Component.js?_dc=1685692706923:3177)
at constructor.prototype.doFireEvent (EventDomain.js?_dc=1685692706927:295)
at constructor.fireEventArgs (Observable.js?_dc=1685692706926:655)
at constructor.fireEvent (Observable.js?_dc=1685692706926:608)
at constructor.processSpecialEvent (Table.js?_dc&#