Error: write after end - ONLY when using 'Content-Type': 'text/html'

const Profile = require("./profile.js");
const renderer = require('./renderer.js');

const commonHeaders = {'Content-Type': 'text/html'};

// Handle HTTP route GET / and POST / i.e. Home
function home(request, response) {
  // if url === "/" && GET
  if (request.url === '/') {
    //show search field
    response.writeHead(200, commonHeaders);
    renderer.view('header', {}, response);
    renderer.view('search', {}, response);
    renderer.view('footer', {}, response);
    response.end();
  }

  // if url === "/" && POST
  // redirect to /:username
}


// Handle HTTP route GET / :username i.e. /ameliagroen
function user(request, response) {
  // if url === "/..."
  const username = request.url.replace('/', '');
  if (username.length > 0) {
    response.writeHead(200, commonHeaders);
    renderer.view('header', {}, response);

    // get json from Treehouse
    const studentProfile = new Profile(username);
    // on 'end'
    studentProfile.on("end", profileJSON => {
      // show profile

      // store values which we need
      const values = {
        avatarURL: profileJSON.gravatar_url,
        username: profileJSON.name,
        badges: profileJSON.badges.length,
        javascriptPoints: profileJSON.points.JavaScript
      }
      // Smple response
      renderer.view('profile', values, response);
      renderer.view('footer', {}, response);
      response.end();

    });
    // on 'error'

    studentProfile.on("error", error => {
      // show error
      renderer.view('error', {errorMessage: error.message}, response);
      renderer.view('search', {}, response);
      renderer.view('footer', {}, response);
      response.end();
    });
  }
}

module.exports.home = home;
module.exports.user = user;

// Function that handles the reading of files and merge in value
const fs = require('fs');

function mergeValues(values, content) {
  // cycle over the keys
  for(let key in values) {
    // replace all {{key}} with the value from the values object
    content = content.replace(`{{${key}}}`, values[key]);
  }
  // return merged content
  return content;
}

function view(templateName, values, response) {
  // read from the template file
  let fileContents = fs.readFileSync(`./views/${templateName}.html`, {encoding: 'utf8'});
  // insert values into the content
  fileContents = mergeValues(values, fileContents);  
  // write out the contents to the response
  response.write(fileContents);
}

module.exports.view = view;

const router = require('./router.js');
// Problem: We need a simple way to look at a user's badge count and JavaScript points from a web browser
// Solution: Use node.js to perform the profile look ups and serve our template via HTTP

// Create a web server
const http = require('http');

const hostname = '127.0.0.1';
const port = 3001;

const server = http.createServer((request, response) => {
  router.home(request, response);
  router.user(request, response);
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});