code: private String resolveKeyWithFallback(String keyName) {
  
          String result = resolveKey(keyName);
          if ((result == null) && (m_keyFallbackHandler != null)) {
              Optional<String> fallback = m_keyFallbackHandler.getFallbackKey(keyName);
              if (fallback.isPresent()) {
                  result = resolveKey(fallback.get());
              }
          }
          return result;
      }
docs: Resolves a message key, using the key fallback handler if it is set.<p>
  
  @param keyName the key to resolve
  
  @return the resolved key