Clustering : NPE on startup

Steps to reproduce
-------------------
1. Start a single controller node with clustering enabled
2. Connect mininet to the controller
3. On the controller console press CTRL+C to exit
4. Exit mininet
5. Start the controller again

You will see the following exception show up.

2014-08-19 21:59:02 PDT [com.google.common.util.concurrent.Futures$ImmediateFuture] SEVERE com.google.common.util.concurrent.Futures$ImmediateFuture addListener RuntimeException while executing runnable com.google.common.util.concurrent.Futures$4@6d11edc3 with executor com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService@3410d526
java.lang.NullPointerException
at org.opendaylight.controller.cluster.datastore.Shard$2.onSuccess(Shard.java:273)
at org.opendaylight.controller.cluster.datastore.Shard$2.onSuccess(Shard.java:271)
at com.google.common.util.concurrent.Futures$4.run(Futures.java:1149)
at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:293)
at com.google.common.util.concurrent.Futures$ImmediateFuture.addListener(Futures.java:99)
at com.google.common.util.concurrent.Futures.addCallback(Futures.java:1152)
at com.google.common.util.concurrent.Futures.addCallback(Futures.java:1088)
at org.opendaylight.controller.cluster.datastore.Shard.commit(Shard.java:271)
at org.opendaylight.controller.cluster.datastore.Shard.applyState(Shard.java:369)
at org.opendaylight.controller.cluster.raft.RaftActor.onReceiveCommand(RaftActor.java:163)