Singleton.cs

using UnityEngine;

public class Singleton<T> : MonoBehaviour where T : Component
{
    private static T instance;
    private static bool isGameEnd;

    public static T Instance
    {
        get
        {
            if (instance == null)
            {
                var objs = FindObjectsOfType<T>();
                if (objs.Length > 0)
                {
                    instance = objs[0];
                }

                if (objs.Length > 1)
                {
                    Debug.LogWarning($"have {objs.Length} {typeof(T)} in the scene");
                }

                if (instance == null && !isGameEnd)
                {
                    var gobject = new GameObject(typeof(T).ToString());
                    instance = gobject.AddComponent<T>();
                }
            }

            return instance;
        }
    }

    private void OnApplicationQuit()
    {
        instance = null;
        isGameEnd = true;
    }
}