#vulnerable code 
                                private <T> Iterable<Key<T>> insert(final DBCollection dbColl, final Iterable<T> entities, final WriteConcern wc) {
        final List<DBObject> list = entities instanceof List
                                    ? new ArrayList<DBObject>(((List<T>) entities).size())
                                    : new ArrayList<DBObject>();

        final Map<Object, DBObject> involvedObjects = new LinkedHashMap<Object, DBObject>();
        for (final T ent : entities) {
            final MappedClass mc = mapper.getMappedClass(ent);
            if (mc.getAnnotation(NotSaved.class) != null) {
                throw new MappingException(
                                              "Entity type: " + mc.getClazz().getName()
                                              + " is marked as NotSaved which means you should not try to save it!");
            }
            list.add(entityToDBObj(ent, involvedObjects));
        }

        final WriteResult wr = null;

        final DBObject[] dbObjects = new DBObject[list.size()];
        dbColl.insert(list.toArray(dbObjects), wc);

        throwOnError(wc, wr);

        final List<Key<T>> savedKeys = new ArrayList<Key<T>>();
        final Iterator<T> entitiesIT = entities.iterator();
        final Iterator<DBObject> dbObjectsIT = list.iterator();

        while (entitiesIT.hasNext()) {
            final T entity = entitiesIT.next();
            final DBObject dbObj = dbObjectsIT.next();
            savedKeys.add(postSaveGetKey(entity, dbObj, dbColl, involvedObjects));
        }

        return savedKeys;
    } 
                            #location 9 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private <T> Iterable<Key<T>> insert(final DBCollection dbColl, final Iterable<T> entities, final WriteConcern wc) {
        //        return save(entities, wc);
        final List<DBObject> list = entities instanceof List
                                    ? new ArrayList<DBObject>(((List<T>) entities).size())
                                    : new ArrayList<DBObject>();

        final Map<Object, DBObject> involvedObjects = new LinkedHashMap<Object, DBObject>();
        for (final T ent : entities) {
            list.add(toDbObject(involvedObjects, ent));
        }

        final WriteResult wr = null;

        final DBObject[] dbObjects = new DBObject[list.size()];
        dbColl.insert(list.toArray(dbObjects), wc);

        throwOnError(wc, wr);

        final List<Key<T>> savedKeys = new ArrayList<Key<T>>();
        final Iterator<T> entitiesIT = entities.iterator();
        final Iterator<DBObject> dbObjectsIT = list.iterator();

        while (entitiesIT.hasNext()) {
            final T entity = entitiesIT.next();
            final DBObject dbObj = dbObjectsIT.next();
            savedKeys.add(postSaveGetKey(entity, dbObj, dbColl, involvedObjects));
        }

        return savedKeys;
    }