code: public static void initialize(String token, ExecutorService executor) {
      if (token == null) {
        throw new IllegalArgumentException("Token should be defined.");
      }
      if (executor == null) {
        throw new IllegalArgumentException("Executor should be defined.");
      }
      initialize(new ClientProperties(token, null, executor));
    }
docs: Initialize client.
  @param token token
  @param executor custom executor service
  @throws IllegalArgumentException when token or executor is {@code null}.