message: [cli] Ensure the `devProcessPort` is set during "upgrade" events
  Related to:
diff: @@ -923,9 +923,17 @@ export default class DevServer {
  await once(this.watcher, 'ready');
  // Configure the server to forward WebSocket "upgrade" events to the proxy.
  - this.server.on('upgrade', (req, socket, head) => {
  + this.server.on('upgrade', async (req, socket, head) => {
  + await this.startPromise;
  + if (!this.devProcessPort) {
  + this.output.debug(
  + `Detected "upgrade" event, but closing socket because no frontend dev server is running`
  + );
  + socket.destroy();
  + return;
  + }
  const target = `http://localhost:${this.devProcessPort}`;
  - this.output.debug(`Detected upgrade event, proxying to ${target}`);
  + this.output.debug(`Detected "upgrade" event, proxying to ${target}`);
  this.proxy.ws(req, socket, head, { target });
  });
  