[MC-48398] Game crashes/freezes on Superflat worldtype generation

When Creating a Superflat Minecraft get's stuck at: Converting world.
No modifications where made to the Superflat settings, Gamemode Creative
[14:17:11 INFO]: Client> s: Exception getting block type in world
[14:17:11 INFO]: Client> 	at ob.a(SourceFile:583) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at cbx.a(SourceFile:80) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at cbx.f(SourceFile:110) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at net.minecraft.server.MinecraftServer.run(SourceFile:357) [14w07a.jar:?]
[14:17:11 INFO]: Client> 	at mr.run(SourceFile:657) [14w07a.jar:?]
[14:17:11 INFO]: Client> Caused by: java.lang.NullPointerException
[14:17:11 INFO]: Client> 	at ri.a(SourceFile:79) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at ri.<init>(SourceFile:22) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.D(SourceFile:2833) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at azb.a(SourceFile:330) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at ayu.a(SourceFile:781) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at ayj.a(SourceFile:36) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aya.a(SourceFile:87) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at atm.a(SourceFile:149) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at oa.a(SourceFile:175) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aso.a(SourceFile:854) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at oa.c(SourceFile:104) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at oa.d(SourceFile:117) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.b(SourceFile:270) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.f(SourceFile:265) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.c(SourceFile:201) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.d(SourceFile:216) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at aja.b(SourceFile:182) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at atg.a(SourceFile:64) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at ob.b(SourceFile:615) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	at ob.a(SourceFile:573) ~[14w07a.jar:?]
[14:17:11 INFO]: Client> 	... 4 more