API call works fine after npm start but crashes at refresh

There is an API call on http://localhost:9000/testAPI.

In bin/www:

var port = normalizePort(process.env.PORT || '9000');
app.set('port', port);

in routes/index.js:

var express = require('express');
var router = express.Router();

/* GET home page. */
router.get('/', function(req, res, next) {
  res.render('index', { title: 'Express' });
});

module.exports = router;

in routes/testAPI.js:

var express = require('express');
var router = express.Router();
var sqlite3 = require('sqlite3').verbose();
var db = new sqlite3.Database('./db/ocs_athletes.db');

router.get('/', function (req, res, next) {
  db.serialize(function () {
    db.all('SELECT athlete_id, name, surname FROM Athlete', function (
      err,
      row
    ) {
     return res.send(row);
    });
  });

  db.close();
});

module.exports = router;

When the command npm start is run, the API works, going to http://localhost:9000/testAPI shows a JSON containing the data.

The problem appears when the page is refreshed. It says Page cannot be reached and in terminal it throws this error:

GET /testAPI 304 20.628 ms - -
undefined:0



Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:526:11)
    at ServerResponse.header (/home/rares/Documents/Projects/ocsApp/api/node_modules/express/lib/response.js:767:10)
    at ServerResponse.json (/home/rares/Documents/Projects/ocsApp/api/node_modules/express/lib/response.js:264:10)
    at ServerResponse.send (/home/rares/Documents/Projects/ocsApp/api/node_modules/express/lib/response.js:158:21)
    at Statement.<anonymous> (/home/rares/Documents/Projects/ocsApp/api/routes/testAPI.js:12:11)
    at Statement.replacement (/home/rares/Documents/Projects/ocsApp/api/node_modules/sqlite3/lib/trace.js:25:27) {
  code: 'ERR_HTTP_HEADERS_SENT'
}
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! api@0.0.0 start: `node ./bin/www`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the api@0.0.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

Is there a way to solve this problem?

This problem appears in this case because of db.close(), that line shouldn't be in the code.

So it would work fine if it is like this:

var express = require('express');
var router = express.Router();
var sqlite3 = require('sqlite3').verbose();
var db = new sqlite3.Database('./db/ocs_athletes.db');

router.get('/', function (req, res, next) {
  db.serialize(function () {
    db.all('SELECT athlete_id, name, surname FROM Athlete', function (
      err,
      row
    ) {
     return res.send(row);
    });
  });
});

module.exports = router;