[CORD-1925] Fix null pointer exception in DHCP relay store

NPE happened when remove DHCP record

Change-Id: I4b4c564d086c4e274aefa0ebce5d52cd622596d0
diff --git a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DistributedDhcpRelayStore.java b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DistributedDhcpRelayStore.java
index 1e87ba6..38c0580 100644
--- a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DistributedDhcpRelayStore.java
+++ b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/store/DistributedDhcpRelayStore.java
@@ -66,7 +66,13 @@
     protected void activated() {
         dhcpRecords = storageService.<HostId, DhcpRecord>eventuallyConsistentMapBuilder()
                 .withName("DHCP-Relay-Records")
-                .withTimestampProvider((hostId, record) -> new WallClockTimestamp(record.lastSeen()))
+                .withTimestampProvider((hostId, record) -> {
+                    if (record != null) {
+                        return new WallClockTimestamp(record.lastSeen());
+                    } else {
+                        return new WallClockTimestamp();
+                    }
+                })
                 .withSerializer(APP_KRYO)
                 .build();
         listener = new InternalMapListener();