Express sub-apps not working

I'm trying to make my main app be able to dynamically add other sub-apps so that I can do something like '/user/appName' where appName is the name of the sub-app. In my project directory I have an 'apps' directory that will contain all the sub-apps (they will all have their own app.js). I made a simple function to do this and it works if I comment out all the error handling stuff. However, I would still like to use error handling.

I call the useApp function from a post method in the main app.

I've search and tried many things, but can't find anything where someone is trying to do something similar. Maybe I'm going about this the wrong way?

app.js

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');

var index = require('./routes');
var login = require('./routes/login');
var user = require('./routes/user');
var admin = require('./routes/administrator');
var networkView = require('./routes/networkView');

var app = express();

//view engine setup
app.set('views', path.join(__dirname, 'views'));

/** if you want hogan to use ".html" file extension
app.engine('html', require('hjs').renderFile);
app.set('view engine', 'html');
**/

app.set('view engine', 'hjs');

// uncomment after placing your favicon in /public
//app.use(favicon(__dirname + '/public/favicon.ico'));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', index);
app.use('/login', login);
app.use('/user', user);
app.use('/admin', admin);
app.use('/networkView', networkView);

// import a sub app
function useApp(appName) {
    app.use('/user/' + appName, require('./apps/' + appName + '/app'));
}

// 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: {}
    });
});

exports.useApp = useApp;
module.exports = app;