Destroy and close the App.

        :return:
            None.

         :note:
            Once destroyed an App can no longer be used.

def destroy(self):
        """
        Destroy and close the App.

        :return:
            None.

         :note:
            Once destroyed an App can no longer be used.
        """
        # 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()