code: boolean addObserver(ApptentiveNotificationObserver observer, boolean useWeakReference) {
  		if (observer == null) {
  			throw new IllegalArgumentException("Observer is null");
  		}
  
  		if (!contains(observer)) {
  			observers.add(useWeakReference ? new ObserverWeakReference(observer) : observer);
  			return true;
  		}
  
  		return false;
  	}
docs: Adds an observer to the list without duplicates.
  
  @param useWeakReference - use weak reference if <code>true</code>
  @return <code>true</code> - if observer was added