code: private static void verifyFeatureKey( String key, String searchedField, IHMProgressMonitor pm ) {
          if (key == null) {
              if (pm != null) {
                  pm.errorMessage(msg.message("trentoP.error.featureKey") + searchedField);
              }
              throw new IllegalArgumentException(msg.message("trentoP.error.featureKey") + searchedField);
          }
  
      }
docs: Verify if there is a key of a FeatureCollections.
  
  @param key
  @throws IllegalArgumentException
  if the key is null.