Start live development even if there is no open file

[dangoor]

We added the ability to find an HTML file and start Live Development with that. If I open a project (like "Getting Started") but have no open files, the Live Preview button complains about wanting me to open an HTML file even though it could find an index.html file at the root.

[marcelgerber]

I just pushed a commit. The main thing is fixed there, but you will get an error message every time you start live development from an project with no open files. The problem is caused because on line 571 doc is null (as console.log(doc) in the next line says).
Maybe anyone of you can try to fix, cause I have to go (night in germany, school tomorrow).

[pthiess]

Assigning to @ingorichter as he's reviewing the PR - low priority.