503 Internal Error

//... imports and package declaration omitted

public class Main
{
    public static void main(String[] args) {

        CourseIdeaDAO dao = new SimpleCourseIdeaDAO();

        staticFileLocation("/public");
        port(8000);

        before((request, response) -> {
            if (request.cookie("username") != null) {
                request.attribute("username", request.cookie("username"));
            }
        });

        before("/ideas", ((request, response) -> {
            // TODO:pjh - send message about redirect somehow
            if (request.cookie("username") == null) {
                response.redirect("/");
                halt();
            }
        }));

        // ... other methods omitted from this forum

        get("/ideas", (request, response) -> {
            Map<String, Object> model = new HashMap<>();
            model.put("ideas", dao.findAll());
            return new ModelAndView(model, "ideas.hbs");
        }, new HandlebarsTemplateEngine());

        post("/ideas", (request, response) -> {
            String title = request.queryParams("title");
            // TODO:pjh - This username is tied to the cookie implementation
            CourseIdea courseIdea = new CourseIdea(title, request.cookie("username"));
            dao.add(courseIdea);
            response.redirect("/ideas");
            return null;
        });
    }

}

public class CourseIdea {
    private String slug;
    private String title;
    private String creator;
    private Set<String> voters;

    public CourseIdea(String title, String creator) {
        this.title = title;
        this.creator = creator;

        Slugify slugify = new Slugify();
        slug = slugify.slugify(title);

    }

    public String getTitle() {
        return title;
    }

    public String getCreator() {
        return creator;
    }

    public String getSlug() { return slug; }

    public boolean addVoter(String voterUserName) {
        return voters.add(voterUserName);
    }

    public int getVoteCount() {
        return voters.size();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        CourseIdea that = (CourseIdea) o;

        if (title != null ? !title.equals(that.title) : that.title != null) return false;
        return creator != null ? creator.equals(that.creator) : that.creator == null;

    }

    @Override
    public int hashCode() {
        int result = title != null ? title.hashCode() : 0;
        result = 31 * result + (creator != null ? creator.hashCode() : 0);
        return result;
    }
}

public class SimpleCourseIdeaDAO implements CourseIdeaDAO {

    private List<CourseIdea> ideas;

    public SimpleCourseIdeaDAO() {
        ideas = new ArrayList<>();
    }

    @Override
    public boolean add(CourseIdea course) {
        return ideas.add(course);
    }

    @Override
    public List<CourseIdea> findAll() {
        return new ArrayList<>(ideas);
    }

    @Override
    public CourseIdea findBySlug(String slug) {
        return ideas.stream()
                .filter(idea -> idea.getSlug().equals(slug))
                .findFirst()
                .orElseThrow(NotFoundException::new);
    }

}