Fixed potential NPE in flow store.

Change-Id: Id4f7887ddf4826f556a62e1a1cd395bc22e882f2
diff --git a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
index b2860b1..368c06d 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/flow/impl/DistributedFlowRuleStore.java
@@ -745,7 +745,7 @@
                             Sets.difference(deviceFlowEntries.keySet(), backedupDevices);
                         if (devicesNotBackedup.size() > 0) {
                             log.warn("Failed to backup devices: {}. Reason: {}",
-                                     devicesNotBackedup, error.getMessage());
+                                     devicesNotBackedup, error != null ? error.getMessage() : "none");
                         }
                         if (backedupDevices != null) {
                             backedupDevices.forEach(id -> {