SingletonBehaviour.cs

using UnityEngine;

public class SingletonBehaviour<T> : MonoBehaviour where T : MonoBehaviour
{
  protected static T s_instance = null;
  public static T Instance
  {
    get
    {
      if (s_instance == null)
      {
        s_instance = FindObjectOfType<T>();
      }
      return s_instance;
    }
  }

  public virtual void Awake()
  {
    if (s_instance == null)
    {
      s_instance = this as T;
    }
  }

  public virtual void OnDestroy()
  {
    if (s_instance == this)
    {
      s_instance = null;
    }
  }
}