#vulnerable code 
                                @Test
    public void simpleDbRefShouldBeSavedAsDbRef() {
        JacksonDBCollection<Owner, String> coll = getCollection(Owner.class, String.class);
        JacksonDBCollection<Referenced, String> refColl = getCollection(Referenced.class, String.class);

        refColl.insert(new Referenced("hello", 10));
        String id = coll.insert(new Owner(new DBRef<Referenced, String>("hello", refColl.getName()))).getSavedId();

        Owner saved = coll.findOneById(id);
        assertThat(saved.ref, notNullValue());
        assertThat(saved.ref.getId(), equalTo("hello"));
        assertThat(saved.ref.getCollectionName(), equalTo(refColl.getName()));

        // Try loading it
        Referenced ref = saved.ref.fetch();
        assertThat(ref, notNullValue());
        assertThat(ref._id, equalTo("hello"));
        assertThat(ref.i, equalTo(10));
    } 
                            #location 17 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void simpleDbRefShouldBeSavedAsDbRef() {
        JacksonDBCollection<Owner, String> coll = getCollection(Owner.class, String.class);
        JacksonDBCollection<Referenced, String> refColl = getCollection(Referenced.class, String.class);

        refColl.insert(new Referenced("hello", 10));
        coll.insert(new Owner(new DBRef<Referenced, String>("hello", refColl.getName())));
        String id = coll.findOne()._id;

        Owner saved = coll.findOneById(id);
        assertThat(saved.ref, notNullValue());
        assertThat(saved.ref.getId(), equalTo("hello"));
        assertThat(saved.ref.getCollectionName(), equalTo(refColl.getName()));

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