django1.9,從github下載xadmin源文件后，跟著視頻配置，然后就出現這個錯誤。RuntimeError: Model class extra_apps.xadmin.models.Bookmark doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. （翻譯：RuntimeError：模型類extra_apps.xadmin.models.Bookmark沒有聲明顯式app_label，也不在INSTALLED_APPS中的應用程序中，或者在加載應用程序之前導入。）