Usually self.app.runner is a reference to the Runner window, but when you close the Runner so there is no widow, it gets set to None. This was a bug in 2020.2.x but it’s one we’ve since fixed, now if there’s no Runner window it opens one, so if you’re comfortable updating to the newest version it should solve this. If not, closing the app and reopening should solve this, until you next close the Runner window without closing the app.