Fix NPE of ProxyArp

Change-Id: I13753586c806ad6beedccf44729971e874ea8e19
diff --git a/apps/proxyarp/src/main/java/org/onosproject/proxyarp/DefaultProxyArp.java b/apps/proxyarp/src/main/java/org/onosproject/proxyarp/DefaultProxyArp.java
index 4f02787..0f613ac 100644
--- a/apps/proxyarp/src/main/java/org/onosproject/proxyarp/DefaultProxyArp.java
+++ b/apps/proxyarp/src/main/java/org/onosproject/proxyarp/DefaultProxyArp.java
@@ -59,8 +59,7 @@
     private ApplicationId appId;
 
     private InternalEdgeListener edgeListener = new InternalEdgeListener();
-    private DefaultNeighbourMessageHandler defaultHandler =
-            new DefaultNeighbourMessageHandler(interfaceService);
+    private DefaultNeighbourMessageHandler defaultHandler;
 
     @Activate
     protected void activate() {
@@ -68,6 +67,8 @@
 
         edgeService.addListener(edgeListener);
         edgeService.getEdgePoints().forEach(this::addDefault);
+
+        defaultHandler = new DefaultNeighbourMessageHandler(interfaceService);
     }
 
     @Deactivate