Singleton.cs

using UnityEngine;
using UnityEngine.Assertions;

// Barebones Singleton class to be used with Unity
// Supports also Singleton instances coming from prefabs (PrefabSingleton). 
// Prefabs need to reside in Resources folder and named [ComponentName].asset.
// If you have GameManager class, then you need Resources/GameManager.asset

public class Singleton<T> : MonoBehaviour where T:MonoBehaviour
{
    // static singleton
    private static T _instance;

    public static T Instance
    {
        get
        {
            if (_instance != null)
            {
                return _instance;
            }
            
            var go = new GameObject(typeof(T).Name);
            // Mark down this instance so it can be accessed
            _instance =  go.AddComponent<T>();
            // Prevent this object from being destroyed when changing scenes
            DontDestroyOnLoad(go);
                        
            return _instance;
        }
    }
}

public class PrefabSingleton<T> : MonoBehaviour where T:MonoBehaviour
{
    // static singleton
    private static T _instance;

    public static T Instance
    {
        get
        {
            if (_instance != null)
            {
                return _instance;
            }

            var prefab = Resources.Load<T>(typeof(T).Name);
            Assert.IsNotNull(prefab, string.Format("prefab {0} in resources not found", typeof(T).Name));
            var go = Instantiate(prefab);
            
            // Mark down this instance so it can be accessed
            _instance =  go.GetComponent<T>();
            Assert.IsNotNull(_instance, string.Format("prefab {0} does not have component", typeof(T).Name));
            
            // Prevent this object from being destroyed when changing scenes
            DontDestroyOnLoad(go);
                        
            return _instance;
        }
    }
}