Fix NPE in PacketManager

This happens when a device event comes in when there
is no device service available.

Change-Id: I50ea44da433979ef0a3a238b22d295e671849ff4
diff --git a/core/net/src/main/java/org/onosproject/net/packet/impl/PacketManager.java b/core/net/src/main/java/org/onosproject/net/packet/impl/PacketManager.java
index 1ecf5b4..943bb93 100644
--- a/core/net/src/main/java/org/onosproject/net/packet/impl/PacketManager.java
+++ b/core/net/src/main/java/org/onosproject/net/packet/impl/PacketManager.java
@@ -452,6 +452,10 @@
         public void event(DeviceEvent event) {
             eventHandlingExecutor.execute(() -> {
                 try {
+                    if (driverService == null) {
+                        // Event came in after the driver service shut down, nothing to be done
+                        return;
+                    }
                     Device device = event.subject();
                     Driver driver = driverService.getDriver(device.id());
                     if (driver == null) {