KAFKA-1890 Fix bug preventing Mirror Maker from successful rebalance; reviewed by Gwen Shapira and Neha Narkhede

core/src/main/scala/kafka/tools/MirrorMaker.scala

@@ -213,11 +213,11 @@ object MirrorMaker extends Logging with KafkaMetricsGroup {
     val customRebalanceListenerClass = options.valueOf(consumerRebalanceListenerOpt)
     val customRebalanceListener = {
       if (customRebalanceListenerClass != null)
-        Utils.createObject[ConsumerRebalanceListener](customRebalanceListenerClass)
+        Some(Utils.createObject[ConsumerRebalanceListener](customRebalanceListenerClass))
       else
-        null
+        None
     }
-    consumerRebalanceListener = new InternalRebalanceListener(mirrorDataChannel, Some(customRebalanceListener))
+    consumerRebalanceListener = new InternalRebalanceListener(mirrorDataChannel, customRebalanceListener)
     connector.setConsumerRebalanceListener(consumerRebalanceListener)
 
     // create producer threads