/// <summary>
 /// Adds the given script as an invoker
 /// </summary>
 /// <param name="invoker">the invoker</param>
 public static void AddInvoker_Pickup(PickupSpawner invoker)
 {
     // add invoker to list and add all listeners to invoker
     invokers_Pickup.Add(invoker);
     foreach (UnityAction <GameObject> listener in listeners_Pickup)
     {
         invoker.AddListener_Pickup(listener);
     }
 }