code: def destroy(self):
          
          # if this is the main_app - set the _main_app class variable to `None`.
          if self == App._main_app:
              App._main_app = None
          self.tk.destroy()
docs: Destroy and close the App.
  
          :return:
              None.
  
           :note:
              Once destroyed an App can no longer be used.