Try & Error

using UnityEngine;

public abstract class MonoSingleton<T> : MonoBehaviour where T : MonoSingleton<T>
{
    protected static bool m_bDontDestroyOnLoad = true;
    
    private static bool m_bApplicationQuit = false;
    private static object InstanceLocker = new object();

    private static T m_Instance = null;
    public static T Instance
    {
        get
        {
            if (true == m_bApplicationQuit)
                return null;

            lock (InstanceLocker)
            {
                // Instance requiered for the first time, we look for it
                if (null == m_Instance)
                {
                    T instance = GameObject.FindObjectOfType(typeof(T)) as T;

                    // Object not found, we create a temporary one
                    if (instance == null)
                    {
                        instance = new GameObject(typeof(T).ToString()).AddComponent<T>();

                        // Problem during the creation, this should not happen
                        if (instance == null)
                        {
                            Debug.LogError("Problem during the creation of " + typeof(T).ToString());
                        }
                    }

                    if (instance != null)
                    {
                        Initialize(instance);
                    }
                }

                return m_Instance;
            }
        }
    }

    private static void Initialize(T instance)
    {
        if (m_Instance == null)
        {
            var startTime = System.DateTime.Now;
            m_Instance = instance;

            // 씬 전환 시, 삭제시킬 싱글톤은 부모 객체에 안붙이도록..
            // 싱글톤 시작 시, m_bDontDestroyOnLoad 셋팅 필요.
            if (true == m_bDontDestroyOnLoad)
            {
                GameObject goRoot = GameObject.Find("Singletons") as GameObject;
                if (null == goRoot)
                {
                    goRoot = new GameObject("Singletons");
                    // DontDestroyOnLoad() 등록은 하위 상속받는 쪽에서 하도록 하는 게 나을까?
                    DontDestroyOnLoad(goRoot);
                }
                m_Instance.transform.parent = goRoot.transform;
            }

            m_Instance.OnInitialize();
            var period = System.DateTime.Now - startTime;
            if (period.TotalSeconds > 1.0f)
            {
                var name = m_Instance.ToString();
                Debug.LogWarning("Profile Warnning. Singletion {" + name + "} too long init time : " 
                                 + period.TotalSeconds.ToString("F") + "Seconds");
            }
        }
        else if (m_Instance != instance)
        {
            DestroyImmediate(instance.gameObject);
        }
    }

    private static void Destroyed(T instance)
    {
        if (m_Instance == instance)
        {
            m_Instance.OnFinalize();
            m_Instance = null;
        }
    }

    public void CreateSingleton() { }
    // [Warning] GameObject에 Component로 미리 등록된 상태에서는 OnInitialize() 호출 안됨.
    public virtual void OnInitialize() { }
    // [Warning] GameObject에 Component로 미리 등록된 상태에서는 OnFinalize() 호출 안됨.
    public virtual void OnFinalize() { }
    protected virtual void CheckDontDestroyOnLoad() { }

    private void Awake()
    {
        Initialize(this as T);
    }

    void OnDestroy()
    {
        Destroyed(this as T);
    }

    private void OnApplicationQuit()
    {
        m_bApplicationQuit = true;
        Destroyed(this as T);
    }
}