code: public static KeenClient client() {
          if (ClientSingleton.INSTANCE.client == null) {
              throw new IllegalStateException("Please call KeenClient.initialize() before requesting the client.");
          }
          return ClientSingleton.INSTANCE.client;
      }
docs: Call this to retrieve the {@code KeenClient} singleton instance.
  
  @return The singleton instance of the client.