Probably happening because of symbolic linking the saves file to my Dropbox folder, but it worked flawlessly in previous versions. The source code probably needs to check if saves is a symbolic link as well, other than just a directory, before trying to create it. Hard linking directories in MacOS compromise fsck, so it has to be avoided.
Error message:
The game crashed whilst initializing game
Error: java.lang.RuntimeException: java.nio.file.FileAlreadyExistsException: /Users/***/Library/Application Support/minecraft/saves
Environment
Error message:
The game crashed whilst initializing game
Error: java.lang.RuntimeException: java.nio.file.FileAlreadyExistsException: /Users/***/Library/Application Support/minecraft/saves
MacOSX 10.13.1, Default Minecraft Launcher, Java Version 8 Update 131
Error message:
The game crashed whilst initializing game
Error: java.lang.RuntimeException: java.nio.file.FileAlreadyExistsException: /Users/***/Library/Application Support/minecraft/saves