POS dropdowns don't work unless there's another annotation layer

[mbollmann]

Loading a file with just a POS tagset linked to it will fail to actually load that tagset; the columns will show up, but the dropdown boxes will be empty.

Adding a "comment" layer fixes that problem (and possibly also other layers, but I haven't tried every possible option), also for files that are subsequently opened that only have a POS tagset. The problem reappears for those files after refreshing the page, suggesting that it's a front-end bug.