(vNet) fix : warning for virtual flow rules

Sometimes virtual flow rule provider complains about an null flow entry.
To remove this warning, handling statements are added.

Change-Id: I8f7802a35032a9ad3ba2c9f6a3394630bc63185b
diff --git a/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/provider/DefaultVirtualFlowRuleProvider.java b/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/provider/DefaultVirtualFlowRuleProvider.java
index 2f203aa..bc519d4 100644
--- a/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/provider/DefaultVirtualFlowRuleProvider.java
+++ b/incubator/net/src/main/java/org/onosproject/incubator/net/virtual/impl/provider/DefaultVirtualFlowRuleProvider.java
@@ -265,6 +265,11 @@
 
     private FlowEntry virtualize(FlowEntry flowEntry) {
         FlowRule vRule = virtualizeFlowRule(flowEntry);
+
+        if (vRule == null) {
+            return null;
+        }
+
         FlowEntry vEntry = new DefaultFlowEntry(vRule, flowEntry.state(),
                                                 flowEntry.life(),
                                                 flowEntry.packets(),
@@ -610,6 +615,11 @@
                 if (frm.isVirtualIngressRule(event.subject())) {
                     NetworkId networkId = frm.getVirtualNetworkId(event.subject());
                     FlowEntry vEntry = getVirtualFlowEntry(event.subject());
+
+                    if (vEntry == null) {
+                        return;
+                    }
+
                     frm.addOrUpdateFlowEntry(networkId, vEntry.deviceId(), vEntry);
 
                     VirtualFlowRuleProviderService providerService =
@@ -628,6 +638,10 @@
                     NetworkId networkId = frm.getVirtualNetworkId(event.subject());
                     FlowEntry vEntry = getVirtualFlowEntry(event.subject());
 
+                    if (vEntry == null) {
+                        return;
+                    }
+
                     frm.removeFlowEntry(networkId, vEntry.deviceId(), vEntry);
                     frm.removeFlowRule(networkId, vEntry.deviceId(), vEntry);