Singleton MonoBehaviour class template which helps to build over when you need Singletons in Unity.

SingletonMonoBehaviour.cs

using UnityEngine;

[DisallowMultipleComponent]
public class SingletonMonoBehaviour : MonoBehaviour
{
    private static SingletonMonoBehaviour m_instance;
    public static SingletonMonoBehaviour Instance
    {
        get
        {
            if (m_instance == null)
            {
                var go = new GameObject(nameof(SingletonMonoBehaviour));
                var comp = go.AddComponent<SingletonMonoBehaviour>();
                m_instance = comp;
            }

            return m_instance;
        }
    }

    private void EnsureSingleExistence()
    {
        if (m_instance != null)
        {
            if (m_instance == this) return;
            
            Destroy(this);
            return;
        }

        m_instance = this;
    }

    private void Awake()
    {
        EnsureSingleExistence();
    }

    private void OnEnable()
    {
        EnsureSingleExistence();
    }

    private void OnDestroy()
    {
        if (m_instance == this)
        {
            m_instance = null;
        }
    }
}