通知中心的NotificationModel没初始化导致空指针崩溃

Log: 修复dock上点击通知中心会崩溃的问题
Change-Id: Ia608084af5d3b842a2e2df7967f1e6c1f848d3db
Reviewed-on: http://gerrit.uniontech.com/c/dde-control-center/+/3089
Reviewed-by: <mailman@uniontech.com>
Reviewed-by: wangwei <wangwei@uniontech.com>
Reviewed-by: fanpengcheng <fanpengcheng@uniontech.com>
Tested-by: <mailman@uniontech.com>