Unity MonoBehaviour Singleton. Base class for creating a singleton in Unity, handles searching for an existing instance, useful if you've created the instance inside a scene. Also handles cleaning up of multiple singleton instances in Awake.

Singleton.cs

// ReSharper disable once InvalidXmlDocComment
/**
 * Singleton.cs
 * Author: Luke Holland (http://lukeholland.me/)
 */

using System;
using UnityEngine;

namespace Koyou.Commons
{
    public abstract class SingletonMonoBehaviour<T> : MonoBehaviour
        where T : MonoBehaviour
    {
        private static T sInstance;
        // ReSharper disable once StaticMemberInGenericType
        private static readonly object sInstanceLock = new();
        // ReSharper disable once StaticMemberInGenericType
        private static bool sQuitting;

        public static T Instance
        {
            get
            {
                lock (sInstanceLock)
                {
                    if (sInstance != null || sQuitting) return sInstance;
                    sInstance = FindObjectOfType<T>();
                    if (sInstance != null) return sInstance;
                    var go = new GameObject(typeof(T).ToString());
                    sInstance = go.AddComponent<T>();

                    DontDestroyOnLoad(sInstance.gameObject);

                    return sInstance;
                }
            }
        }

        protected virtual void Awake()
        {
            if (sInstance == null) sInstance = gameObject.GetComponent<T>();
            else if (sInstance.GetInstanceID() != GetInstanceID())
            {
                Destroy(gameObject);
                throw new Exception($"Instance of {GetType().FullName} already exists, removing {ToString()}");
            }
        }

        protected virtual void OnApplicationQuit()
        {
            sQuitting = true;
        }
    }
}