code: public DirectoryPollerBuilder addPolledDirectory(PolledDirectory directory) {
          if (directory == null) {
              throw new NullPointerException(NULL_ARGUMENT_ERROR_MESSAGE);
          }
          directories.add(directory);
          return this;
      }
docs: Adds the given <code>directory</code> to the list of polled directories.
  Mandatory to add at least one directory.
  <p>
  Once the {@link DirectoryPoller} has been built, it can be used to add
  additional polled directories, or remove polled directories.
  
  @param directory - the directory to poll.
  @throws NullPointerException - if the given argument is null.
  @return {@link DirectoryPollerBuilder}