_resetOpenDocumentIDs should always return a promise

@timothynoel found that clicking `File > Share on Behance...` reliably crashed Node. The problem was that in the reentrant path through `DocumentManager._resetOpenDocumentIDs` no promise was returned, whereas the rest of the code assumed that this method would always return a promise. (We should probably rewrite all of this in TypeScript to prevent mistakes like this. Or get a better JavaScript programmer.)

Please test, review and merge into ps-next ASAP, @joelrbrandt :cocktail: