Singleton to return only on instance of NotificationManager.
        :returns: instance of NotificationManager

def instance():
        """
        Singleton to return only on instance of NotificationManager.
        :returns: instance of NotificationManager
        """

        if not hasattr(NotificationManager, '_instance') or NotificationManager._instance is None:
            NotificationManager._instance = NotificationManager()
        return NotificationManager._instance