Singleton utility file for Unity from Tarodev ( https://www.youtube.com/watch?v=tE1qH8OxO2Y&t=229s )

Singleton.cs

using UnityEngine;

public abstract class StaticInstance<T> : MonoBehaviour where T : MonoBehaviour
{
    public static T Instance { get; private set; }
    protected virtual void Awake() => Instance = this as T;

    protected void OnApplicationQuit()
    {
        Instance = null;
        Destroy(gameObject);
    }
}

public abstract class Singleton<T> : StaticInstance<T> where T : MonoBehaviour
{
    protected override void Awake()
    {
        if (Instance != null) Destroy(gameObject);
        base.Awake();
    }
}

public abstract class PersistentSingleton<T> : Singleton<T> where T : MonoBehaviour
{
    protected override void Awake()
    {
        base.Awake();
        DontDestroyOnLoad(gameObject);
    }
}