Logging information about device not being driver programmable instead of throwing exception

Change-Id: I1d64e83ca0cabc9378bcaf19badddbba584128f8
diff --git a/core/net/src/main/java/org/onosproject/net/group/impl/GroupDriverProvider.java b/core/net/src/main/java/org/onosproject/net/group/impl/GroupDriverProvider.java
index 75fbb89..35f3ba0 100644
--- a/core/net/src/main/java/org/onosproject/net/group/impl/GroupDriverProvider.java
+++ b/core/net/src/main/java/org/onosproject/net/group/impl/GroupDriverProvider.java
@@ -16,6 +16,7 @@
 
 package org.onosproject.net.group.impl;
 
+import org.onosproject.net.Device;
 import org.onosproject.net.DeviceId;
 import org.onosproject.net.device.DeviceService;
 import org.onosproject.net.group.GroupOperations;
@@ -60,10 +61,12 @@
     }
 
     private GroupProgrammable getGroupProgrammable(DeviceId deviceId) {
-        GroupProgrammable programmable = deviceService.getDevice(deviceId).as(GroupProgrammable.class);
-        if (programmable == null) {
+        Device device = deviceService.getDevice(deviceId);
+        if (device.is(GroupProgrammable.class)) {
+            return device.as(GroupProgrammable.class);
+        } else {
             log.warn("Device {} is not group programmable");
+            return null;
         }
-        return programmable;
     }
 }