Fix: resolve a set of NPE at kubevirt networking

Change-Id: I9ff05b3def4b60614a012ae2b9b97eb2b77e1d67
(cherry picked from commit 5f0b38a6eab3b4c5bffc09f23bb78fcdf641281c)
diff --git a/apps/kubevirt-networking/app/src/main/java/org/onosproject/kubevirtnetworking/impl/NetworkAttachmentDefinitionWatcher.java b/apps/kubevirt-networking/app/src/main/java/org/onosproject/kubevirtnetworking/impl/NetworkAttachmentDefinitionWatcher.java
index d3a8ee4..eff1ca4 100644
--- a/apps/kubevirt-networking/app/src/main/java/org/onosproject/kubevirtnetworking/impl/NetworkAttachmentDefinitionWatcher.java
+++ b/apps/kubevirt-networking/app/src/main/java/org/onosproject/kubevirtnetworking/impl/NetworkAttachmentDefinitionWatcher.java
@@ -258,7 +258,9 @@
             log.trace("Process NetworkAttachmentDefinition {} removal event from API server.",
                     name);
 
-            adminService.removeNetwork(name);
+            if (adminService.network(name) != null) {
+                adminService.removeNetwork(name);
+            }
         }
 
         private boolean isMaster() {