=clu #17712 Guard against readView not created

* readView might be null if Cluster init fails before it is created,
  i.e. shutdown is called from constructor

akka-cluster/src/main/scala/akka/cluster/Cluster.scala

@@ -362,6 +362,9 @@ class Cluster(val system: ExtendedActorSystem) extends Extension {
       logInfo("Shutting down...")
 
       system.stop(clusterDaemons)
+
+      // readView might be null if init fails before it is created
+      if (readView != null)
         readView.close()
 
       closeScheduler()