title: Possible Nullpointer in spring-kafka
body: <p>we are using <strong>spring-kafka-2.0.1.RELEASE</strong> and get this error in production:</p>
  
  <blockquote>
    <p>exception java.lang.NullPointerException
    java.lang.NullPointerException: null at
    org.springframework.kafka.listener.KafkaMessageListenerContainer$ListenerConsumer.access$000(KafkaMessageListenerContainer.java:271)
    ~[spring-kafka-2.0.1.RELEASE.jar:2.0.1.RELEASE] at
    org.springframework.kafka.listener.KafkaMessageListenerContainer.getAssignedPartitions(KafkaMessageListenerContainer.java:152)
    ~[spring-kafka-2.0.1.RELEASE.jar:2.0.1.RELEASE] at</p>
  </blockquote>
  
  <p>we are using following props:</p>
  
  <pre><code>Map&lt;String, Object&gt; props = new HashMap&lt;&gt;();
  props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, host);
  props.put(ConsumerConfig.GROUP_ID_CONFIG, groupId);
  props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, autoCommit);
  props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, sessionTimeout);
  props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
  props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, EspDLPResponseDeserializer.class);
  props.put(ConsumerConfig.RECONNECT_BACKOFF_MS_CONFIG, reconnectMs); // when esp server unreachable, default value too small
  props.put(ConsumerConfig.RETRY_BACKOFF_MS_CONFIG, retryBackoff); // default value too small
  props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, maxPollRec);
  
  if (saslEnabled) {
  props.put(SaslConfigs.SASL_MECHANISM, "PLAIN");
  props.put(CommonClientConfigs.SECURITY_PROTOCOL_CONFIG, "SASL_SSL");
  // jaas.conf with -D server start parameter
  props.put(SaslConfigs.SASL_JAAS_CONFIG, jaas_config);
  props.put(SslConfigs.SSL_TRUSTSTORE_LOCATION_CONFIG, ssl_keystore);
  }
  </code></pre>
  
  <p>Can you please check?</p>
  
  <p>Thank you very much</p>