Hello,
there seems to be a problem between railcraft and thaumcraft.

Used versions:

• minecraft 1.6.4
• forge 9.11.1.938
• railcraft 8.2.0.0
• thaumcraft 4.0.3a

Here is the error log. It occured entering a mystcraft world an it seems to be a retrogen thing, cause the world already exists:

[2013-11-05 09:40:05] net.minecraft.util.ReportedException: Exception getting block type in world
[2013-11-05 09:40:05] at net.minecraft.world.World.func_72798_a(World.java:423)
[2013-11-05 09:40:05] at net.minecraft.world.gen.feature.WorldGenMinable.func_76484_a(WorldGenMinable.java:79)
[2013-11-05 09:40:05] at thaumcraft.common.lib.world.ThaumcraftWorldGenerator.generateOres(ThaumcraftWorldGenerator.java:452)
[2013-11-05 09:40:05] at thaumcraft.common.lib.world.ThaumcraftWorldGenerator.generateSurface(ThaumcraftWorldGenerator.java:315)
[2013-11-05 09:40:05] at thaumcraft.common.lib.world.ThaumcraftWorldGenerator.worldGeneration(ThaumcraftWorldGenerator.java:73)
[2013-11-05 09:40:05] at thaumcraft.common.lib.WorldTicker.tickEnd(WorldTicker.java:126)
[2013-11-05 09:40:05] at cpw.mods.fml.common.SingleIntervalHandler.tickEnd(SingleIntervalHandler.java:34)
[2013-11-05 09:40:05] at cpw.mods.fml.common.FMLCommonHandler.tickEnd(FMLCommonHandler.java:141)
[2013-11-05 09:40:05] at cpw.mods.fml.common.FMLCommonHandler.onPostWorldTick(FMLCommonHandler.java:282)
[2013-11-05 09:40:05] at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:677)
[2013-11-05 09:40:05] at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:587)
[2013-11-05 09:40:05] at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:175)
[2013-11-05 09:40:05] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
[2013-11-05 09:40:05] at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)
[2013-11-05 09:40:05] Caused by: java.lang.NullPointerException
[2013-11-05 09:40:05] at mods.railcraft.common.worldgen.GeodePopulator.canGen(GeodePopulator.java:63)
[2013-11-05 09:40:05] at mods.railcraft.common.worldgen.GeodePopulator.generateGeode(GeodePopulator.java:52)
[2013-11-05 09:40:05] at mods.railcraft.common.worldgen.GeodePopulator.generate(GeodePopulator.java:46)
[2013-11-05 09:40:05] at net.minecraftforge.event.ASMEventHandler_91_GeodePopulator_generate_Pre.invoke(.dynamic)
[2013-11-05 09:40:05] at net.minecraftforge.event.ASMEventHandler.invoke(ASMEventHandler.java:39)
[2013-11-05 09:40:05] at net.minecraftforge.event.EventBus.post(EventBus.java:108)
[2013-11-05 09:40:05] at biomesoplenty.world.ChunkProviderBOP.func_73153_a(ChunkProviderBOP.java:576)
[2013-11-05 09:40:05] at net.minecraft.world.gen.ChunkProviderServer.func_73153_a(ChunkProviderServer.java:258)
[2013-11-05 09:40:05] at net.minecraft.world.chunk.Chunk.func_76624_a(Chunk.java:1253)
[2013-11-05 09:40:05] at net.minecraft.world.gen.ChunkProviderServer.func_73158_c(ChunkProviderServer.java:153)
[2013-11-05 09:40:05] at net.minecraft.world.gen.ChunkProviderServer.func_73154_d(ChunkProviderServer.java:166)
[2013-11-05 09:40:05] at net.minecraft.world.World.func_72964_e(World.java:529)
[2013-11-05 09:40:05] at net.minecraft.world.World.func_72798_a(World.java:414)
[2013-11-05 09:40:05] ... 13 more

Someone created a biome without a name, shame on them.

I'll add a check to prevent crashing when other mods derp that up.

Thank you very much