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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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()