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]