Fixing NPE in GossipDeviceStore.

Change-Id: I093bf498487bb48ea70f2a3c2443ef57d60600d1
diff --git a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
index 03a8673..4e6c05d 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
@@ -494,16 +494,18 @@
     }
 
     private DeviceEvent markOfflineInternal(DeviceId deviceId, Timestamp timestamp) {
-
         Map<ProviderId, DeviceDescriptions> providerDescs
                 = getOrCreateDeviceDescriptionsMap(deviceId);
 
         // locking device
         synchronized (providerDescs) {
-
             // accept off-line if given timestamp is newer than
             // the latest Timestamp from Primary provider
             DeviceDescriptions primDescs = getPrimaryDescriptions(providerDescs);
+            if (primDescs == null) {
+                return null;
+            }
+
             Timestamp lastTimestamp = primDescs.getLatestTimestamp();
             if (timestamp.compareTo(lastTimestamp) <= 0) {
                 // outdated event ignore