SingletonMonoBehaviour.cs

using System;
using UnityEngine;

public abstract class SingletonMonoBehaviour<T> : MonoBehaviour where T : SingletonMonoBehaviour<T> {
    [SerializeField] protected bool DontDestroy = false;

    private static T _instance;

    public static bool HasInstance => _instance != null;

    public static T I {
        get {
            // if singleton already exists, return it
            if (_instance) {
                return _instance;
            }

            _instance = FindObjectOfType<T>();

            // if found instance, return it
            if (_instance) {
                return _instance;
            }

            // if not found instance, create log error
            Debug.LogError($"Not found {typeof(T)}");
            return null;
        }
    }

    private void Initialize() {
        var obj = gameObject;

        // if singleton is not this, destroy this instance
        if (I != this) {
            Debug.LogWarning("Destroy this instance for singleton");
            Destroy(obj);
            return;
        }

        if (DontDestroy) {
            DontDestroyOnLoad(obj);
        }

        // rename this gameObject for emphasize singleton
        if (obj.name.IndexOf("[Singleton]", StringComparison.Ordinal) < 0) {
            obj.name = $"[Singleton] {obj.name}";
        }
    }

    protected virtual void Awake() {
        Initialize();
    }
}