Getting An "Cannot convert undefined or null to object" Error

var express = require('express');
var router = express.Router();
var User = require('../models/user');
var mid = require('../middleware');


// GET /profile
router.get('/profile', mid.requiresLogin, function(req, res, next) {
  User.findById(req.session.userId)
      .exec(function (error, user) {
        if (error) {
          return next(error);
        } else {
          return res.render('profile', { title: 'Profile', name: user.name, favorite: user.favoriteBook });
        }
      });
});

//GET /logout
router.get('/logout', function(req, res, next){
  if(req.session){
    // Clear the cookie on the client
    res.clearCookie('connect.sid', { path: '/' });
    // Delet session object
    req.session.destroy(function(err){
      if(err){
        return next(err);
      } else{
        return res.redirect('/');
      }
    });
  }
});

// Get /login
router.get('/login', mid.loggedOut, function(req, res, next) {
  return res.render('login', { title: 'Sign Up' });
});

// Post /login
router.post('/login', function(req, res, next) {
    if(req.body.email && req.body.password) {
        User.authenticate(req.body.email, req.body.password, function (error, user) {
            if (error || !user) {
                var err = new Error('Wrong emial or password');
                err.status = 401;
                return next(err);
            } else {
                req.session.userId = user._id;
                return res.redirect('/profile');
            }
        });
    } else {
        var err = new Error('Email and password are required.')
        err.status = 400;
        return next(err);
    }
});

// Get /register
router.get('/register', mid.loggedOut, function(req, res, next) {
  return res.render('register', { title: 'Log In' });
});

// Post /register
router.post('/register', function(req, res, next) {
    if (req.body.email &&
       req.body.name &&
       req.body.favoriteBook &&
       req.body.password &&
       req.body.confirmPassword) {
        //confirm that user typed same password
        if (req.body.password !== req.body.confirmPassword) {
            var err = new Error('Passwords must match.')
            err.status = 400;
            return next(err);
        }

        //create object with form input
        var userData = {
            email: req.body.email,
            name: req.body.name,
            favoriteBook: req.body.favoriteBook,
            password: req.body.password
        };

        //use schma's create method to insert document into Mongo
        User.create(userData, function (error, user) {
            if(error) {
                return next(error);
            } else {
                req.session.userId = user._id;
                return res.redirect('/profile')
            }
        });

    } else {
        var err = new Error('All fields required.')
        err.status = 400;
        return next(err);
    }
});

// GET /
router.get('/', function(req, res, next) {
  return res.render('index', { title: 'Home' });
});

// GET /about
router.get('/about', function(req, res, next) {
  return res.render('about', { title: 'About' });
});

// GET /contact
router.get('/contact', function(req, res, next) {
  return res.render('contact', { title: 'Contact' });
});

module.exports = router;

var express = require('express');
var bodyParser = require('body-parser');
var mongoose = require('mongoose');
var session = require('express-session');
var MongoStore = require('connect-mongo')(session);
var app = express();



//mongodb connection
mongoose.connect("mongodb://localhost:27017/bookworm");
var db = mongoose.connection;
//mongo error
db.on('error', console.error.bind(console, 'connection error:'))

//use session for tracking login
app.use(session({
  secret: 'treehouse loves you',
  resave: true,
  saveUninitialized: false,
  store: new MongoStore({
      mongooseConnection: db
  })
}));

//make user ID avaiable in templates
app.use(function (req, res, next) {
    res.locals.currentUser = req.session.userId;
    next();
});

// parse incoming requests
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));

// serve static files from /public
app.use(express.static(__dirname + '/public'));

// view engine setup
app.set('view engine', 'pug');
app.set('views', __dirname + '/views');

// include routes
var routes = require('./routes/index');
app.use('/', routes);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('File Not Found');
  err.status = 404;
  next(err);
});

// error handler
// define as the last app.use callback
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
    error: {}
  });
});

// listen on port 3000
app.listen(3000, function () {
  console.log('Express app listening on port 3000');
});