Fixing a couple of NPEs

Change-Id: I19b5a2787b26e67e05640f6c64a13b58f2dc089a
diff --git a/src/main/java/org/onosproject/segmentrouting/config/DeviceProperties.java b/src/main/java/org/onosproject/segmentrouting/config/DeviceProperties.java
index 9d4d884..632eeec 100644
--- a/src/main/java/org/onosproject/segmentrouting/config/DeviceProperties.java
+++ b/src/main/java/org/onosproject/segmentrouting/config/DeviceProperties.java
@@ -93,8 +93,10 @@
      *
      * @param deviceId device identifier
      * @return a map that contains all subnet-to-ports mapping of given device
+     * @throws DeviceConfigNotFoundException
      */
-    Map<Ip4Prefix, List<PortNumber>> getSubnetPortsMap(DeviceId deviceId);
+    Map<Ip4Prefix, List<PortNumber>> getSubnetPortsMap(DeviceId deviceId)
+            throws DeviceConfigNotFoundException;
 
     /**
      * Returns the VLAN cross-connect configuration.