code: public <E> E lookUpWorkingObjectOrReturnNull(E externalObject) {
          if (externalObject == null) {
              return null;
          }
          LookUpStrategy lookUpStrategy = lookUpStrategyResolver.determineLookUpStrategy(externalObject);
          return lookUpStrategy.lookUpWorkingObjectIfExists(idToWorkingObjectMap, externalObject);
      }
docs: As defined by {@link ScoreDirector#lookUpWorkingObjectOrReturnNull(Object)}.
  @param externalObject sometimes null
  @return null if externalObject is null or if there is no workingObject for externalObject
  @throws IllegalArgumentException if it cannot be looked up or if the externalObject's class is not supported
  @throws IllegalStateException if it cannot be looked up
  @param <E> the object type