Getting stuck in Converting world when trying to create a new world (Flatworld, Creative)

Created 7 Creative Flatworld worlds, 5 of them made me stuck in the Converting world screen.

(Also happening if creating a new Snapshot profile with a new game directory)

Console Log:

[13:17:52 INFO]: Client> [13:17:52] [Server thread/INFO]: Stopping server
[13:17:52 INFO]: Client> [13:17:52] [Server thread/INFO]: Saving players
[13:17:52 INFO]: Client> [13:17:52] [Server thread/INFO]: Saving worlds
[13:17:52 INFO]: Client> [13:17:52] [Server thread/INFO]: Saving chunks for level 'New World'/Overworld
[13:17:53 INFO]: Client> [13:17:53] [Server thread/ERROR]: Exception stopping the server
[13:17:53 INFO]: Client> java.lang.NullPointerException
[13:17:53 INFO]: Client> at net.minecraft.server.MinecraftServer.o(SourceFile:330) ~[14w07a.jar:?]
[13:17:53 INFO]: Client> at cbx.o(SourceFile:274) ~[14w07a.jar:?]
[13:17:53 INFO]: Client> at net.minecraft.server.MinecraftServer.run(SourceFile:419) [14w07a.jar:?]
[13:17:53 INFO]: Client> at mr.run(SourceFile:657) [14w07a.jar:?]