private void OnDestroy()
 {
     // unset the singleton reference if it is the current instance
     if (_instance == this)
     {
         _instance = null;
     }
 }
        private void Awake()
        {
            // check if an object already exists
            if (_instance != null)
            {
                throw new Exception("Only a single instance of 'FirstResponder` should exist at a time.");
            }

            _instance = this;
        }