code: protected String getStringOrNull(ResourceBundle bundle, String key) {
          if (bundle.containsKey(key)) {
              try {
                  return bundle.getString(key);
              } catch (MissingResourceException ex) {
                  // Assume key not found for some other reason
                  // -> do NOT throw the exception to allow for checking parent message source.
              }
          }
          return null;
      }
docs: Efficiently retrieve the String value for the specified key,
  or return {@code null} if not found.
  
  @param bundle the ResourceBundle to perform the lookup in
  @param key the key to look up
  @return the associated value, or {@code null} if none
  @see ResourceBundle#getString(String) ResourceBundle#getString(String)
  @see ResourceBundle#containsKey(String) ResourceBundle#containsKey(String)