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

using UnityEngine;

public class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{

    private static T _instance;
    private static readonly object _instanceLock = new();
    private static bool _quitting;

    public static T Instance
    {
        get
        {
            
            lock (_instanceLock)
            {
                if (_instance is not null || _quitting)
                {
                    return _instance;
                }
                
                _instance = FindObjectOfType<T>();

                if (_instance is not null)
                {
                    return _instance;
                }
                
                GameObject go = new(typeof(T).ToString());
                _instance = go.AddComponent<T>();

                DontDestroyOnLoad(_instance.gameObject);

                return _instance;
            }
        }
    }

    protected virtual void Awake()
    {
        
        if (_instance is null)
        {
            _instance = gameObject.GetComponent<T>();
        }
        else if (_instance.GetInstanceID() != GetInstanceID())
        {
            Destroy(gameObject);
        }
    }

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