code: String getFirstTopicId(final File ditaTopicFile) {
          assert ditaTopicFile.isAbsolute();
          if (!ditaTopicFile.isAbsolute()) {
              return null;
          }
          final StringBuilder firstTopicId = new StringBuilder();
          final TopicIdParser parser = new TopicIdParser(firstTopicId);
          try {
              final XMLReader reader = getXMLReader();
              reader.setContentHandler(parser);
              reader.parse(ditaTopicFile.toURI().toString());
          } catch (final RuntimeException e) {
              throw e;
          } catch (final Exception e) {
              logger.error(e.getMessage(), e);
          }
          if (firstTopicId.length() == 0) {
              return null;
          }
          return firstTopicId.toString();
      }
docs: Get the first topic id from the given dita file.
  
  @param ditaTopicFile a dita file.
  @return The first topic id from the given dita file if success, otherwise
  {@code null} string is returned.