Status: Fixed this, and any other bugs resulting from doing stuff without a level loaded, for the next update; by making it impossible to not have a level loaded.
If you start the editor and press F2 without loading a level/creating a new level, the error message reads:
QuoteError: Playtest Mode only supported for Lemmix-based or Lemmini-based styles.
While logically correct, this does not really describe the cause of the error.
Noted. The correct solution is to disable that option altogether when no level is loaded - or better yet, do not allow a situation where no level is loaded.
For the next release, it's (hopefully) impossible to get a situation where no level is currently loaded (when the editor first starts up, it'll have a blank level rather than no level at all).