public Optional<KafkaException> error() {
        return error;
    }
Event that signifies that the application thread has executed the {@link ConsumerRebalanceListener} callback. If
the callback execution threw an error, it is included in the event should any event listener want to know.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/events/ConsumerRebalanceListenerCallbackCompletedEvent.java