Thanks for pointing that out, I was just about to file an issue regarding TOC and missing preamble. Right now, I'm a bit unsure about adding a TOC setting for the editor at all. For one, it's a nice feature for the user who just wants to see a TOC for easy navigation of the document (which does not work if document has no preamble, though).

On the other hand, it interferes with TOC attributes set directly in the document. I think the whole TOC area might need a bit of cleanup.