On line number 228 in Node.java
SubscriptionsExtension subElem = (SubscriptionsExtension) reply.getExtension(PubSubElementType.SUBSCRIPTIONS);
where
PubSubElementType.SUBSCRIPTIONS("subscriptions", PubSubNamespace.BASIC),
But for getSubscriptionsAsOwner it should have been
SUBSCRIPTIONS("subscriptions", PubSubNamespace.OWNER),
I guess thats why subElem is null.