I have seen this error in both 2.7.3 and 2.9.0 when connecting to a replica set that seems to be up and running and in good status.

java.lang.NullPointerException
at com.mongodb.OutMessage.query(OutMessage.java:79)
at com.mongodb.OutMessage.query(OutMessage.java:75)
at com.mongodb.DBPort.findOne(DBPort.java:142)
at com.mongodb.DBPort.runCommand(DBPort.java:148)
at com.mongodb.ConnectionStatus$UpdatableNode.update(ConnectionStatus.java:184)
at com.mongodb.ReplicaSetStatus$UpdatableReplicaSetNode.update(ReplicaSetStatus.java:614)
at com.mongodb.ReplicaSetStatus$Updater.updateAll(ReplicaSetStatus.java:766)
at com.mongodb.ReplicaSetStatus$Updater.run(ReplicaSetStatus.java:736)

We have seen this error occur intermittently which will quickly fill up our logs and roll them continuously. It is often seen with the "server seen down" error, but then this error takes over the logs.