code: public void removeDataModelListener(DataModelListener listener) {
  
          if (listener == null) {
              throw new NullPointerException();
          }
          if (listeners != null) {
              listeners.remove(listener);
              if (listeners.isEmpty()) {
                  listeners = null;
              }
          }
  
      }
docs: <p>Remove an existing {@link DataModelListener} from the set
  interested in notifications from this {@link DataModel}.</p>
  
  @param listener The old {@link DataModelListener} to be deregistered
  
  @throws NullPointerException if <code>listener</code>
  is <code>null</code>