MonoSingleton.cs

using UnityEngine;
using System;

public class MonoSingleton<T> : MonoBehaviour where T : class
{
    /// <summary>
    /// Makes the object singleton not be destroyed automatically when loading a new scene.
    /// </summary>
    public bool DontDestroy;

    private static MonoSingleton<T> _instance;

    public static T Instance => _instance as T;

    protected virtual void Awake()
    {
        if (_instance != null)
        {
            Destroy(gameObject);
        }
        else
        {
            _instance = this;

            if (DontDestroy)
            {
                DontDestroyOnLoad(gameObject);
            }
            
            Initialize();
        }
    }

    protected virtual void Initialize()
    {
    }

    protected virtual void Shutdown()
    {
    }

    protected virtual void OnDestroy()
    {
        if (_instance == this)
        {
            _instance = null;
            Shutdown();
        }
    }

    protected virtual void OnApplicationQuit()
    {
        if (_instance == this)
        {
            _instance = null;
            Shutdown();
        }
    }
}