code: public <T> void register(Class<T> event, EventObserver<T> observer){
          if(event == null){
              throw new IllegalArgumentException("Null Event type passed to register");
          }
          if(observer == null){
              throw new IllegalArgumentException("Null observer passed to register");
          }
          nullSafeGet(event).add(observer);
      }
docs: Register the given observer to be triggered if the given event type is triggered.
  
  @param event type
  @param observer event observer
  @param <T> relating type