The nep.getSipId() should be checked if it is null.

Change-Id: I6b52c45f9a9f797b2c89da24cd0158396e13d662
diff --git a/apps/odtn/service/src/main/java/org/onosproject/odtn/internal/DefaultTapiResolver.java b/apps/odtn/service/src/main/java/org/onosproject/odtn/internal/DefaultTapiResolver.java
index f309d6d..6167377 100644
--- a/apps/odtn/service/src/main/java/org/onosproject/odtn/internal/DefaultTapiResolver.java
+++ b/apps/odtn/service/src/main/java/org/onosproject/odtn/internal/DefaultTapiResolver.java
@@ -97,7 +97,7 @@
     public boolean hasNepRef(String sipId) {
         updateCache();
         return tapiNepRefList.stream()
-                .anyMatch(nep -> nep.getSipId().equals(sipId));
+                .anyMatch(nep -> nep.getSipId() != null && nep.getSipId().equals(sipId));
     }
 
     @Override