Any estimate yet on when 1.13 will be released, though? I've been playing with snapshots before and I guess I have been lucky my game world has never been corrupted. But after the warning for the recent snapshots, I thought about it, and the fact is that I would be pretty devastated if I lost my game at this point. Put too much time into it. So I'm gonna stop playing snapshots and wait for stable releases only. But now I can't play at all because I was playing on one of the other 1.13 snapshots and I assume going back to 1.12 will break it. Ugh, hopefully fairly soon.
First, always backup your world before loading a new snapshot.
Until the snapshots stabilize, or maybe even until 1.13 is released, I'm keeping my "real" world on 17w46a; I'm still testing out the new snapshots, but only on *copies* of that world (or new worlds entirely). Set the version on your profile to "snapshot 17w46a" (or whichever snapshot your world is on) instead of "latest snapshot" and keep on playing.