Generating a superflat world with the water world preset doesn't work. it gets stuck on Loading World. I left it for an over an hour and nothing happened.

Create New World
Game Mode: Creative
Game Type:superflat
Preset: water world

No response. From the log:
15:04:11 cxk Starting integrated minecraft server version 18w09a
15:04:11 cxk Generating keypair
15:04:12 net.minecraft.server.MinecraftServer Found new data pack vanilla, loading it automatically
15:04:12 uc Reloading ResourceManager: Default
15:04:12 aro Loaded 519 recipes
15:04:13 m Loaded 562 advancements
15:04:13 net.minecraft.server.MinecraftServer Encountered an unexpected exception
java.lang.NullPointerException
at aup.a(SourceFile:420)
at biy.e(SourceFile:60)
at biy.<init>(SourceFile:46)
at bio.o(SourceFile:46)
at rz.o(SourceFile:719)
at rz.<init>(SourceFile:127)
at cxk.a(SourceFile:96)
at cxk.d(SourceFile:131)
at net.minecraft.server.MinecraftServer.run(SourceFile:502)
at java.lang.Thread.run(Thread.java:745)
15:04:13 net.minecraft.server.MinecraftServer This crash report has been saved to: /Users/secret/Library/Application Support/minecraft/crash-reports/crash-2018-03-03_15.04.13-server.txt
15:04:13 net.minecraft.server.MinecraftServer Stopping server
15:04:13 net.minecraft.server.MinecraftServer Saving players
15:04:13 net.minecraft.server.MinecraftServer Saving worlds

Desert preset is fine
Snow preset is fine
normal superflat is fine