Node.js Express Handlears 'No default engine was specified'

I'm making something in Node.js using some of the tutorials in Web Development with Node & Express: Leveraging the JavaScript Stack by Ethan Brown. my index.js looks like this:

var express = require('express')

var app = express();
var handlebars = require('express-handlebars')
  .create({ defaultLayout:'main' });
app.engine('handlebars', handlebars.engine);
app.set('view engine', 'handlebars');

var app = express();

app.set('port', process.env.PORT || 3000);


app.get('/', function(req, res) {
  res.render('mainpage', { title: '메인 페이지' });
});


//404
app.use(function(req, res, next){
  res.type('text/plain');
  res.status(404);
  res.send('404 Error');
});
//500
app.use(function(err, req, res, next){
  res.type('text/plain');
  res.status(500);
  res.send('500 Error');
});

//Section Divider

app.listen(app.get('port'), function(){
  console.log('ㅁㄴㅇㄹ');
});

and it gives me this kind of error if I remove 404 and 500 message code:

Error: No default engine was specified and no extension was provided. at new View (/home/-----/Desktop/DariBB/node_modules/express/lib/view.js:62:11) at EventEmitter.render (/home/-----/Desktop/DariBB/node_modules/express/lib/application.js:570:12) at ServerResponse.render (/home/-----/Desktop/DariBB/node_modules/express/lib/response.js:966:7) at /home/-----/Desktop/DariBB/index.js:15:7 at Layer.handle [as handle_request] (/home/-----/Desktop/DariBB/node_modules/express/lib/router/layer.js:95:5) at next (/home/-----/Desktop/DariBB/node_modules/express/lib/router/route.js:137:13) at Route.dispatch (/home/-----/Desktop/DariBB/node_modules/express/lib/router/route.js:112:3) at Layer.handle [as handle_request] (/home/-----/Desktop/DariBB/node_modules/express/lib/router/layer.js:95:5) at /home/-----/Desktop/DariBB/node_modules/express/lib/router/index.js:281:22 at Function.process_params (/home/-----/Desktop/DariBB/node_modules/express/lib/router/index.js:335:12)

via glacialHCM