NPE in PolicyManager

The following exception occurred when testing a fix for GBP-2 and running the groupbasedpolicy POC demo:

2014-09-15 18:56:17.912 PDT [pool-25-thread-2] ERROR o.o.g.r.ofoverlay.PolicyManager - Failed to write flow table GroupTable
java.lang.NullPointerException: null
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.flow.GroupTable.update(GroupTable.java:81) ~[bundlefile:na]
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.PolicyManager$SwitchFlowUpdateTask.call(PolicyManager.java:322) [bundlefile:na]
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.PolicyManager$SwitchFlowUpdateTask.call(PolicyManager.java:305) [bundlefile:na]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_65]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_65]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:178) [na:1.7.0_65]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:292) [na:1.7.0_65]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_65]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_65]
at java.lang.Thread.run(Thread.java:745) [na:1.7.0_65]
2014-09-15 18:56:17.950 PDT [pool-25-thread-4] ERROR o.o.g.r.ofoverlay.PolicyManager - Failed to write flow table GroupTable
java.lang.NullPointerException: null
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.flow.GroupTable.update(GroupTable.java:81) ~[bundlefile:na]
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.PolicyManager$SwitchFlowUpdateTask.call(PolicyManager.java:322) [bundlefile:na]
at org.opendaylight.groupbasedpolicy.renderer.ofoverlay.PolicyManager$SwitchFlowUpdateTask.call(PolicyManager.java:305) [bundlefile:na]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_65]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_65]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_65]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:178) [na:1.7.0_65]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:292) [na:1.7.0_65]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_65]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_65]
at java.lang.Thread.run(Thread.java:745) [na:1.7.0_65]

This wasn't observed in previous versions, but there should be a NULL pointer check to guard against this.