code: @Override
      public List<Object> getProperty(String key) {
          List<?> nodes = fetchNodeList(key);
  
          if (nodes.size() == 0) {
              return null;
          } else {
              List<Object> list = new ArrayList<>();
              for (Object node : nodes) {
                  ConfigurationNode configurationNode = (ConfigurationNode) node;
                  if (configurationNode.getValue() != null) {
                      list.add(configurationNode.getValue());
                  }
              }
  
              if (list.size() < 1) {
                  return null;
              } else {
                  return list;
              }
          }
      }
docs: Fetches the specified property. This task is delegated to the associated
  expression engine.
  
  @param key the key to be looked up
  @return the found value