NPE on addOrDelDefFibRouteToSNAT when updating ExternalNetwork

This seems to happen when 2 conditions are met:
1. Creating a router associated with the external network on creation. Doesn't happen if first creating the router, and then associating with the network.
2. There are routers which don't have any DPNs associated with them.

This causes the following NPE in ExternalNetworkListener:
2016-09-10 09:39:14,330 | ERROR | dispatcher-64903 | DataChangeListener | 188 - org.opendaylight.controller.sal-distributed-datastore - 1.4.0.SNAPSHOT | Error notifying listener org.opendaylight.netvirt.natservice.internal.Ex
java.lang.NullPointerException
at org.opendaylight.netvirt.natservice.internal.ExternalNetworkListener.addOrDelDefFibRouteToSNAT(ExternalNetworkListener.java:145)[333:org.opendaylight.netvirt.natservice-impl:0.3.0.SNAPSHOT]
at org.opendaylight.netvirt.natservice.internal.ExternalNetworkListener.update(ExternalNetworkListener.java:99)[333:org.opendaylight.netvirt.natservice-impl:0.3.0.SNAPSHOT]
at org.opendaylight.netvirt.natservice.internal.ExternalNetworkListener.update(ExternalNetworkListener.java:40)[333:org.opendaylight.netvirt.natservice-impl:0.3.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.updateData(AbstractDataChangeListener.java:86)[288:org.opendaylight.genius.mdsalutil-api:0.1.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.onDataChanged(AbstractDataChangeListener.java:55)[288:org.opendaylight.genius.mdsalutil-api:0.1.0.SNAPSHOT]
at org.opendaylight.controller.md.sal.binding.impl.AbstractForwardedDataBroker$TranslatingDataChangeInvoker.onDataChanged(AbstractForwardedDataBroker.java:143)[159:org.opendaylight.controller.sal-binding-broker-impl:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.dataChanged(DataChangeListener.java:71)[188:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.handleReceive(DataChangeListener.java:43)[188:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.common.actor.AbstractUntypedActor.onReceive(AbstractUntypedActor.java:26)[183:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at akka.actor.UntypedActor$$anonfun$receive$1.applyOrElse(UntypedActor.scala:165)[171:com.typesafe.akka.actor:2.4.7]
at akka.actor.Actor$class.aroundReceive(Actor.scala:484)[171:com.typesafe.akka.actor:2.4.7]
at akka.actor.UntypedActor.aroundReceive(UntypedActor.scala:95)[171:com.typesafe.akka.actor:2.4.7]
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)[171:com.typesafe.akka.actor:2.4.7]
at akka.actor.ActorCell.invoke(ActorCell.scala:495)[171:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)[171:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.run(Mailbox.scala:224)[171:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.exec(Mailbox.scala:234)[171:com.typesafe.akka.actor:2.4.7]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[167:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[167:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[167:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[167:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]