I've finally managed to install acme/hello-world and got the extension running..
And then.. when I tried the next step of this "flarum-cli init" to create my new js to begin this tutorial (https://docs.flarum.org/extend/frontend), that's when everything crashed.
But that's cool, cause the npm thingy made everything worked again 😃
npm run dev
I then realized, that my acme/hello-world PLUS all the other default extensions in Flarum are gone?
Is there an explanation of what happened and if I did any step wrongly?