Instantiate plugin threads after deserialiser

This moves the instantiation of the plugin thread objects after the
creation of the deserialiser. Previously, the threads were trying to
access the deserialiser which wasn't initialised yet and were trying to
work with a null pointer.

Resolves: OpenTSDB#10

src/main/java/net/opentsdb/tsd/KafkaRpcPluginGroup.java

@@ -129,12 +129,6 @@ public KafkaRpcPluginGroup(final KafkaRpcPlugin parent, final String groupID) {
             : KafkaRpcPluginConfig.DEFAULT_CONSUMER_THREADS;
     kafka_consumers = new ArrayList<KafkaRpcPluginThread>(num_threads);
 
-    for (int i = 0; i < num_threads; i++) {
-      kafka_consumers.add(new KafkaRpcPluginThread(this, i, topics));
-    }
-
-    timer.newTimeout(this, config.threadCheckInterval(), TimeUnit.MILLISECONDS);
-
     final String deser_class = config.getString(
         KafkaRpcPluginConfig.PLUGIN_PROPERTY_BASE + groupID + ".deserializer");
     if (Strings.isNullOrEmpty(deser_class)) {
@@ -179,6 +173,12 @@ public KafkaRpcPluginGroup(final KafkaRpcPlugin parent, final String groupID) {
       throw new IllegalArgumentException("Unable to find a deserializer "
           + "for class [" + deser_class + "]");
     }
+
+    for (int i = 0; i < num_threads; i++) {
+      kafka_consumers.add(new KafkaRpcPluginThread(this, i, topics));
+    }
+
+    timer.newTimeout(this, config.threadCheckInterval(), TimeUnit.MILLISECONDS);
   }
 
   @Override