If modelObject is empty here, it may cause exception later.

Change-Id: I892fdde7dabbf9dc55a74a6722413e260d0eedae
diff --git a/apps/odtn/api/src/main/java/org/onosproject/odtn/utils/tapi/TapiObjectHandler.java b/apps/odtn/api/src/main/java/org/onosproject/odtn/utils/tapi/TapiObjectHandler.java
index 15208a2..9b36a2d 100644
--- a/apps/odtn/api/src/main/java/org/onosproject/odtn/utils/tapi/TapiObjectHandler.java
+++ b/apps/odtn/api/src/main/java/org/onosproject/odtn/utils/tapi/TapiObjectHandler.java
@@ -240,6 +240,9 @@
         if (modelObjectData.modelObjects().size() > 1) {
             throw new IllegalStateException("Multiple modelObject found.");
         }
+        if (modelObjectData.modelObjects().isEmpty()) {
+            throw new IllegalStateException("ModelObject must not be empty.");
+        }
         return (T) modelObjectData.modelObjects().get(0);
     }