code: private int indexOf(ApptentiveNotificationObserver observer) {
  		for (int i = 0; i < observers.size(); ++i) {
  			final ApptentiveNotificationObserver other = observers.get(i);
  			if (other == observer) {
  				return i;
  			}
  
  			final ObserverWeakReference otherReference = ObjectUtils.as(other, ObserverWeakReference.class);
  			if (otherReference != null && otherReference.get() == observer) {
  				return i;
  			}
  		}
  		return -1;
  	}
docs: Returns an index of the observer or its weak reference.
  
  @return -1 if not found