Clustering: NullPointerException when modifying data

Tested on Boron.

1. Install a 3-node ODL cluster with a Netconf device configured on a Netconf connector.
2. Create a piece of data on the device using the Netconf connector.
3. Attempt to modify the data.

The first two steps will go without an error but the third step (modifying the data) will most likely end with a NullPointerException error on transaction commit:

{"errors":{"error":[{"error-type":"application","error-tag":"operation-failed","error-message":"Transaction commit failed","error-info":"java.util.concurrent.ExecutionException: java.lang.NullPointerException\n\tat com.google.common.util.concurrent.AbstractFuture$Sync.getValue(AbstractFuture.java:299)\n\tat com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:286)\n\tat com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:116)\n\tat com.google.common.util.concurrent.ForwardingFuture.get(ForwardingFuture.java:63)\n\tat com.google.common.util.concurrent.AbstractCheckedFuture.checkedGet(AbstractCheckedFuture.java:78)\n\tat org.opendaylight.netconf.sal.restconf.impl.RestconfImpl.updateConfigurationData(RestconfImpl.java:723)\n\tat org.opendaylight.netconf.sal.restconf.impl.StatisticsRestconfServiceWrapper.updateConfigurationData(StatisticsRestconfServiceWrapper.java:133)\n\tat org.opendaylight.netconf.sal.rest.impl.RestconfCompositeWrapper.updateConfigurationData(RestconfCompositeWrapper.java:82)\n\tat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)\n\tat sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)\n\tat sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)\n\tat java.lang.reflect.Method.invoke(Method.java:497)\n\tat com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)\n\tat com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$ResponseO [truncated]