code: protected void removeBehaviorListener(BehaviorListener listener) {
  
          if (listener == null) {
              throw new NullPointerException();
          }
          if (listeners == null) {
              return;
          }
          listeners.remove(listener);
  
          clearInitialState();
      }
docs: <p class="changed_added_2_0">Remove the specified
  {@link BehaviorListener} from the set of listeners
  registered to receive event notifications from this
  {@link Behavior}.
  
  @param listener The {@link BehaviorListener} to be deregistered
  @throws NullPointerException if <code>listener</code>
  is <code>null</code>
  
  @since 2.0