SingletonMonoBehaviour.cs

   /// <summary>
    /// シングルトンパターン（MonoBehaviour）
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public abstract class SingletonMonoBehaviour<T> : MonoBehaviour where T : MonoBehaviour
    {
        private static T _instance;
        private static object _lock = new object();
        // アプリが終了しているかどうかのフラグ
        private static bool applicationIsQuitting = false;
        /// <summary>
        /// シングルトンインスタンス
        /// </summary>
        public static T Instance
        {
            get
            {
                if (applicationIsQuitting)
                {
                    Debug.LogWarning("[Singleton] Instance '" + typeof(T) +
                                     "' already destroyed on application quit." +
                                     " Won't create again - returning null.");
                    return null;
                }

                lock (_lock)
                {
                    if (_instance == null)
                    {
                        _instance = (T) FindObjectOfType(typeof(T));

                        if (FindObjectsOfType(typeof(T)).Length > 1)
                        {
                            Debug.LogError("[Singleton] Something went really wrong " +
                                           " -there should never be more than 1 singleton!" +
                                           " Reopening the scene may fix it.");
                            return _instance;
                        }

                        if (_instance == null)
                        {
                            GameObject singleton = new GameObject();
                            _instance = singleton.AddComponent<T> ();
                            singleton.name = "(singleton)" + typeof(T).ToString();
                            DontDestroyOnLoad(singleton);
                            Debug.Log("[Singleton] An instance of " + typeof(T) +
                                      " is needed in the scene, so '" + singleton +
                                      "' was created with DontDestroyOnLoad.");
                        }
                        else
                        {
                            Debug.Log("[Singleton] Using instance already created: " +
                                      _instance.gameObject.name);
                        }
                    }

                    return _instance;
                }
            }
        }

        public virtual void OnDestroy()
        {
            applicationIsQuitting = true;
        }
    }