web-contents-created fired too early?

main.js

const {app, BrowserWindow, webContents: WebContents} = require('electron')
const assert = require('assert')

let mainWindow

function createWindow () {
  mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    webPreferences: {
      nodeIntegration: true
    }
  })

  mainWindow.loadFile('index.html')

  mainWindow.on('closed', function () {
    mainWindow = null
  })
}

app.on('web-contents-created', (e, webContents) => {
  const theId = webContents.id
  const existingWebContents = WebContents.fromId(theId)
  assert(!!existingWebContents, 'webContents missing?!')
})

app.on('ready', createWindow)

app.on('window-all-closed', function () {
  app.quit()
})

app.on('activate', function () {
  if (mainWindow === null) {
    createWindow()
  }
})