#fixed code
                                @Test
    public void testObjectIdAnnotationOnStringGenerated() {
        StringId object = new StringId();

        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);

        coll.insert(object);
        String id = coll.findOne()._id;
        // Check that it's a valid object id
        assertTrue(org.bson.types.ObjectId.isValid(id));
        StringId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
        assertThat(coll.getDbCollection().findOne().get("_id").toString(), equalTo(id));
    }
#vulnerable code 
                                @Test
    public void testObjectIdAnnotationOnStringGenerated() {
        StringId object = new StringId();

        JacksonDBCollection<StringId, String> coll = getCollection(StringId.class, String.class);

        String id = coll.insert(object).getSavedId();
        // Check that it's a valid object id
        assertTrue(org.bson.types.ObjectId.isValid(id));
        StringId result = coll.findOneById(id);
        assertThat(result._id, equalTo(id));
    } 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE