#vulnerable code 
                                @Test
    public void dbRefWithObjectIdShouldBeSavedAsDbRef() {
        JacksonDBCollection<ObjectIdOwner, String> coll = getCollection(ObjectIdOwner.class, String.class);
        JacksonDBCollection<ObjectIdReferenced, byte[]> refColl = getCollection(ObjectIdReferenced.class, byte[].class);

        byte[] refId = refColl.insert(new ObjectIdReferenced(10)).getSavedId();
        String id = coll.insert(new ObjectIdOwner(new DBRef<ObjectIdReferenced, byte[]>(refId, refColl.getName()))).getSavedId();

        ObjectIdOwner saved = coll.findOneById(id);
        assertThat(saved.ref, notNullValue());
        assertThat(saved.ref.getId(), equalTo(refId));
        assertThat(saved.ref.getCollectionName(), equalTo(refColl.getName()));

        // Try loading it
        ObjectIdReferenced ref = saved.ref.fetch();
        assertThat(ref, notNullValue());
        assertThat(ref._id, equalTo(refId));
        assertThat(ref.i, equalTo(10));
    } 
                            #location 10 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void dbRefWithObjectIdShouldBeSavedAsDbRef() {
        JacksonDBCollection<ObjectIdOwner, String> coll = getCollection(ObjectIdOwner.class, String.class);
        JacksonDBCollection<ObjectIdReferenced, byte[]> refColl = getCollection(ObjectIdReferenced.class, byte[].class);

        byte[] refId = new org.bson.types.ObjectId().toByteArray();
        refColl.insert(new ObjectIdReferenced(refId, 10));
        coll.insert(new ObjectIdOwner(new DBRef<ObjectIdReferenced, byte[]>(refId, refColl.getName())));
        String id = coll.findOne()._id;

        ObjectIdOwner saved = coll.findOneById(id);
        assertThat(saved.ref, notNullValue());
        assertThat(saved.ref.getId(), equalTo(refId));
        assertThat(saved.ref.getCollectionName(), equalTo(refColl.getName()));

        // Try loading it
        ObjectIdReferenced ref = saved.ref.fetch();
        assertThat(ref, notNullValue());
        assertThat(ref._id, equalTo(refId));
        assertThat(ref.i, equalTo(10));
    }