将数据保存到mongoDB后，NodeJS服务器崩溃

我编写了一个脚本，使用mongoose将对象保存到MongoDB数据库。对象被正确地保存到数据库，但服务器崩溃后，抛出我以下错误消息:catch(err) { process.nextTick(function() { throw err}); }

这是我的一部分代码:

users.js

var User = require('../models/user');
router.post('/register', function(req, res, next) {
[...]
// checks for errors
var errors = req.validationErrors();
if (errors) {
    res.render('register', {
        errors: errors,
        [...]
    })
} else {
    var newUser = new User({
      [...]
    });
    // Create user
    User.createUser(newUser, function(err, user) {
        if (error) {
          throw err;
        };
        console.log(user);
    });
    [...]
}
});
module.exports = router;

user.js

var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/nodeauth');
// User Schema
var UserSchema = mongoose.Schema({
    [...]
});
var User = module.exports = mongoose.model('User', UserSchema);
module.exports.createUser = function(newUser, callback) {
    newUser.save(callback);
}

apps.js

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var passport = require('passport');
var localStrategy = require('passport-local').Strategy;
var bodyParser = require('body-parser');
var multer = require('multer');
var flash = require('connect-flash');
var mongo = require('mongodb');
var mongoose = require('mongoose');
var db = mongoose.connection;
var expressValidator = require('express-validator');
var routes = require('./routes/index');
var users = require('./routes/users');
var app = express();
// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
// handle file uploads
app.use(multer({dest:'./uploads'}).single('singleInputFileName'));
// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
// handle express session
app.use(session({
secret: 'secret',
saveUninitialized: true,
resave: true
}));
// passport
app.use(passport.initialize());
app.use(passport.session());
// validator
app.use(expressValidator({
  errorFormatter: function(param, msg, value) {
  var namespace = param.split('.')
  , root    = namespace.shift()
  , formParam = root;
while(namespace.length) {
  formParam += '[' + namespace.shift() + ']';
}
return {
  param : formParam,
  msg   : msg,
  value : value
};
  }
    }));
    app.use(cookieParser());
    app.use(express.static(path.join(__dirname, 'public')));
// connect flash
app.use(flash());
app.use(function (req, res, next) {
  res.locals.messages = require('express-messages')(req, res);
  next();
});
app.use('/', routes);
app.use('/users', users);
// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});
// error handlers
// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
  app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
      message: err.message,
      error: err
    });
  });
}
// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
    error: {}
  });
});

module.exports = app;