Found the bug, and it's exactly what it looked like from your side: once you'd deleted every post, there was no way left to delete the discussion itself.
Here's why. The "..." menu (the one with "Delete discussion" in it) isn't attached to the discussion. It's rendered as part of the first post. Delete that post, and everything else in the thread, and there's nothing left for the page to hang that menu on, so it just doesn't show up. The discussion is still sitting in the database with zero posts attached to it. That's the ghost state you ended up in: visible on the home screen, no button anywhere to remove it.
Nothing was stopping you from getting there, either. Deleting a post never checked whether it happened to be the first and last one in its thread. Removing your own posts one by one was a normal, allowed action that just happened to end in a dead end.
Fixed now: deleting a discussion's first post deletes the whole discussion. There's no in-between state anymore where a discussion exists with no posts in it. Turned out there was a related gap too: a moderator removing someone else's opening post, or an author blocked by a delete time limit, used to go through the plain "delete post" action instead, which skipped the discussion-level rules entirely. Both paths now go through the same check
This will ship in the next release. I'll update this thread once it's out.