Description: Exception ticking world

java.lang.NullPointerException: Exception ticking world
	at atw.a(SourceFile:207)
	at atw.a(SourceFile:136)
	at sa.k_(SourceFile:198)
	at net.minecraft.server.MinecraftServer.w(SourceFile:695)
	at net.minecraft.server.MinecraftServer.v(SourceFile:628)
	at cvl.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:533)
	at java.lang.Thread.run(Thread.java:748)

minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block;12;