SONAR suggestion - Synchronize initialization of static fields

Change-Id: I2ddf8220bda49f235ddfc5713174c11a147df354
diff --git a/incubator/protobuf/services/nb/src/main/java/org/onosproject/incubator/protobuf/services/nb/GrpcNbDeviceService.java b/incubator/protobuf/services/nb/src/main/java/org/onosproject/incubator/protobuf/services/nb/GrpcNbDeviceService.java
index b583800..a06acf6 100644
--- a/incubator/protobuf/services/nb/src/main/java/org/onosproject/incubator/protobuf/services/nb/GrpcNbDeviceService.java
+++ b/incubator/protobuf/services/nb/src/main/java/org/onosproject/incubator/protobuf/services/nb/GrpcNbDeviceService.java
@@ -52,7 +52,7 @@
 
     private final Logger log = getLogger(getClass());
 
-    private static DeviceServiceNbServerInternal instance = null;
+    private DeviceServiceNbServerInternal instance = null;
 
     @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
     protected GrpcServiceRegistry registry;