Singleton.cs

using JetBrains.Annotations;
using UnityEngine;

namespace TinyMatter.Core {
public abstract class Singleton<T> : Singleton where T : MonoBehaviour
{
    #region Fields
    [CanBeNull]
    private static T _instance;

    [NotNull]
    // ReSharper disable once StaticMemberInGenericType
    private static readonly object Lock = new object();

    [SerializeField]
    private bool _persistent = true;
    
    #endregion

    #region Properties
    [NotNull]
    public static T Instance
    {
        get
        {
            if (Quitting)
            {
                // ReSharper disable once AssignNullToNotNullAttribute
                return null;
            }
            lock (Lock)
            {
                if (_instance != null)
                    return _instance;
                var instances = FindObjectsOfType<T>();
                var count = instances.Length;
                if (count > 0)
                {
                    if (count == 1) {
                        return _instance = instances[0];
                    }


                    for (var i = 1; i < instances.Length; i++) {
                        Destroy(instances[i]);
                    }

                    return _instance = instances[0];
                }

                return _instance = new GameObject($"({nameof(Singleton)}){typeof(T)}").AddComponent<T>();
            }
        }
    }
    #endregion

    #region Methods
    private void Awake()
    {
        if (_persistent)
            DontDestroyOnLoad(gameObject);
        OnAwake();
    }

    protected virtual void OnAwake() { }
    #endregion
}

public abstract class Singleton : MonoBehaviour
{
    #region Properties
    public static bool Quitting { get; private set; }
    #endregion

    #region  Methods
    private void OnApplicationQuit()
    {
        Quitting = true;
    }
    #endregion
}
//Free candy!
}