>Hm.. how about throw "Item was no journal article"; ?

`throw new Error('Item was not a journal article.');` (maybe give a list of extracted types so it's easier to troubleshoot/improve. unless you already know what we're not handling, in which case, make a comment in the code)

>But in this way the multiple picking is not working at all if you also select a non-article (e.g. all select all), while at the moment it works for all selected article.

Yes, that's what I would expect. If a user selected something, we have to import it or inform the user that something went wrong. Silently failing is not a good idea. If there is no way to figure out from ToC what would be imported and what would not, I would say to not give the user an option to import from ToC at all.