code: public void addHttpSenderListener(HttpSenderListener httpSenderListener) {
          if (httpSenderListener == null) {
              throw new IllegalArgumentException("Parameter httpSenderListener must not be null.");
          }
  
          if (httpSenderListeners == null) {
              httpSenderListeners = new ArrayList<>();
          }
          httpSenderListeners.add(httpSenderListener);
      }
docs: Adds the given {@code httpSenderListener} to the extension hook, to be later added to the
  {@link org.parosproxy.paros.network.HttpSender HttpSender}.
  <p>
  By default, the {@code HttpSenderListener}s added to this extension hook are removed from the {@code HttpSender} when the
  extension is unloaded.
  
  @param httpSenderListener the HttpSenderListener that will be added to the {@code HttpSender}
  @throws IllegalArgumentException if the given {@code httpSenderListener} is {@code null}.
  @since 2.7.0