docs: private List<Object> getSourceFieldValues(TransformationStep step) throws Exception {
          List<Object> sources = new ArrayList<Object>();
          for (String sourceField : step.getSourceFields()) {
              Object object = getObjectValue(sourceField, true);
              if (object == null) {
                  String message = String.format("The source field %s is null. Step will be be ignored", sourceField);
                  throw new TransformationStepException(message);
              }
              sources.add(object);
          }
          return sources;
      }