Dev-Server returns empty page

[PJaros]

• Running Ubuntu 18.04
• Installed yarn according to https://yarnpkg.com/lang/en/docs/install/#debian-stable
  git clone https://github.com/blinkenrocket/webedit-react
  cd webedit-react
  yarn
  yarn build
  yarn dev
• With google-chrome/firefox accessing http://127.0.0.1:8080/ shows a white page

I can't find any error messages on the dev-webserver window or on the browser console.