Can you please assist am getting an error: 'void ' type not allowed here post ("/ideas", (req, res) -> {

package com.teamtreehouse.courses;

import com.teamtreehouse.courses.model.CourseIdea;
import com.teamtreehouse.courses.model.CourseIdeaDAO;
import com.teamtreehouse.courses.model.SimpleCourseIdeaDAO;
import spark.ModelAndView;
import spark.template.handlebars.HandlebarsTemplateEngine;

import java.util.HashMap;
import java.util.Map;

import static spark.Spark.*;

public class Main {
    public static void main(String[] args) {
        staticFileLocation("/public");
        CourseIdeaDAO dao = new SimpleCourseIdeaDAO();

        get("/", (req, res) -> {
            Map<String, String> model = new HashMap<>();
            model.put("username", req.cookie("username"));
            return new ModelAndView(model, "index.hbs");
        }, new HandlebarsTemplateEngine());

        post("/sign-in", (req, res) -> {
            Map<String, String> model = new HashMap<>();
            String username = req.queryParams("username");
            res.cookie("username", username);
            model.put("username", username);
            return new ModelAndView(model, "/.");
        }, //new HandlebarsTemplateEngine());


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

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