STC fixes

- NPE when a switch has already been removed and an attempt is made
  to access stats on the switch.

- NPE in group provider if the system is in the process of shutting down.

- Lower ERROR to WARN when switch resources are not available.

Change-Id: I5b90ff328b19c2d661fde3a24e90614999c4c900
diff --git a/providers/openflow/flow/src/main/java/org/onosproject/provider/of/flow/impl/OpenFlowRuleProvider.java b/providers/openflow/flow/src/main/java/org/onosproject/provider/of/flow/impl/OpenFlowRuleProvider.java
index bbf1436..16fca36 100644
--- a/providers/openflow/flow/src/main/java/org/onosproject/provider/of/flow/impl/OpenFlowRuleProvider.java
+++ b/providers/openflow/flow/src/main/java/org/onosproject/provider/of/flow/impl/OpenFlowRuleProvider.java
@@ -269,6 +269,10 @@
         Dpid dpid = Dpid.dpid(flowRule.deviceId().uri());
         OpenFlowSwitch sw = controller.getSwitch(dpid);
 
+        if (sw == null) {
+            return;
+        }
+
         FlowRuleExtPayLoad flowRuleExtPayLoad = flowRule.payLoad();
         if (hasPayload(flowRuleExtPayLoad)) {
             OFMessage msg = new ThirdPartyMessage(flowRuleExtPayLoad.payLoad());
@@ -298,6 +302,10 @@
         Dpid dpid = Dpid.dpid(flowRule.deviceId().uri());
         OpenFlowSwitch sw = controller.getSwitch(dpid);
 
+        if (sw == null) {
+            return;
+        }
+
         FlowRuleExtPayLoad flowRuleExtPayLoad = flowRule.payLoad();
         if (hasPayload(flowRuleExtPayLoad)) {
             OFMessage msg = new ThirdPartyMessage(flowRuleExtPayLoad.payLoad());