Expected router outlet to be a valid DOM Node

[y2us]

On Firefox Ubuntu

I use npm start and then the content doesn't appear, only the menu.

Uncaught (in promise) TypeError: [Vaadin.Router] Expected router outlet to be a valid DOM Node (but got undefined)
__ensureOutlet router.js:752
__addAppearingContent router.js:786
ready router.js:498
promise callback*render router.js:469
attachRouter index.ts:29
firstUpdated app-index.ts:103
_$didUpdate reactive-element.ts:1350
performUpdate reactive-element.ts:1317
scheduleUpdate reactive-element.ts:1235
__enqueueUpdate reactive-element.ts:1207
requestUpdate reactive-element.ts:1182
_initialize reactive-element.ts:924
ReactiveElement reactive-element.ts:907
LitElement lit-element.js:82
AppIndex app-index.ts:18
legacyCustomElement custom-element.ts:21
customElement custom-element.ts:64
__decorateClass app-index.ts:7
app-index.ts:18
router.js:752:12
GEThttp://192.168.1.108:8000/src/pages/page-about.ts
[HTTP/1.1 304 Not Modified 1ms]

[abdonrd]

Sorry for the late response!
Looks like there is an issue in the latest @web/dev-server-esbuild.
PR fixing it: #516