Affects Version/s: Minecraft 1.13-pre1
Fix Version/s: Minecraft 1.13-pre2
When clicking "Singleplayer" in the main menu the game freezes for about 10 seconds and outputs various warnings and errors in the launcher log.
I have various old worlds that were last opened in various old versions in the saves folders and it seems that it fails loading the level.dat of some of them.
I don't think the level.dats of these worlds are corrupted though, because in 1.12.2 they load without any issues.
It seems there are two outputs that might relate to the freeze/fail of loading the level.dats.
14 times something like this:
Likely relates to/overlaps with
And 10 times something like this (it fails for level.dat as well as for level.dat_old, so it fails for five worlds):