argument should be checked for null before dereferencing

Change-Id: I3f13d924727a0e522f149d0e1842a730fe7d0e14
diff --git a/providers/isis/topology/src/main/java/org/onosproject/provider/isis/topology/impl/IsisTopologyProvider.java b/providers/isis/topology/src/main/java/org/onosproject/provider/isis/topology/impl/IsisTopologyProvider.java
index 1ff2c33..8989d64 100644
--- a/providers/isis/topology/src/main/java/org/onosproject/provider/isis/topology/impl/IsisTopologyProvider.java
+++ b/providers/isis/topology/src/main/java/org/onosproject/provider/isis/topology/impl/IsisTopologyProvider.java
@@ -62,6 +62,7 @@
 import java.util.List;
 import java.util.StringTokenizer;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static org.slf4j.LoggerFactory.getLogger;
 
 /**
@@ -151,6 +152,7 @@
      * @return link description
      */
     private LinkDescription buildLinkDes(IsisLink isisLink) {
+        checkNotNull(isisLink);
         long srcAddress = 0;
         long dstAddress = 0;
         boolean localPseduo = false;
@@ -185,9 +187,8 @@
         ConnectPoint src = new ConnectPoint(srcId, PortNumber.portNumber(srcAddress));
         ConnectPoint dst = new ConnectPoint(dstId, PortNumber.portNumber(dstAddress));
         DefaultAnnotations.Builder annotationBuilder = DefaultAnnotations.builder();
-        if (isisLink != null) {
-            annotationBuilder = buildAnnotations(annotationBuilder, isisLink);
-        }
+
+        annotationBuilder = buildAnnotations(annotationBuilder, isisLink);
 
         return new DefaultLinkDescription(src, dst, Link.Type.DIRECT, false, annotationBuilder.build());
     }