Unity singleton class. Just inherit.

Singleton.cs

public class Singleton<T>: MonoBehaviour where T: MonoBehaviour {
   protected static T instance;

   // Returns the instance of this singleton.
   public static T Instance {
      get {
         if (instance == null) {
            instance = (T)FindObjectOfType(typeof(T));
 
            if (instance == null) {
               Debug.LogError("An instance of " + typeof(T) + " is needed in the scene, but there is none.");
            }
         }

         return instance;
      }
   }
}