Try explicit class casting to remove warning message

Change-Id: If7830a12e0405c5b37f6e864bd7dfb915871959f
diff --git a/web/api/src/main/java/org/onosproject/rest/resources/NetworkConfigWebResource.java b/web/api/src/main/java/org/onosproject/rest/resources/NetworkConfigWebResource.java
index a39d81c..5c593a0 100644
--- a/web/api/src/main/java/org/onosproject/rest/resources/NetworkConfigWebResource.java
+++ b/web/api/src/main/java/org/onosproject/rest/resources/NetworkConfigWebResource.java
@@ -152,7 +152,7 @@
                 nullIsNotFound(service.getConfigClass(subjectClassKey, configKey),
                                configKeyNotFoundErrorString(subjectClassKey, subjectKey, configKey));
         Config config =
-                nullIsNotFound(service.getConfig(subject, configClass),
+                (Config) nullIsNotFound(service.getConfig(subject, configClass),
                                configKeyNotFoundErrorString(subjectClassKey,
                                                             subjectKey,
                                                             configKey));