I get those NullPointerExceptions with Biomes o Plenty 0.55 and Gregtech 3.07g
Happend for me during creation of my world on a server but the game starts fine even with this errors.

013-06-16 22:17:45 [INFO] [STDERR] java.lang.NullPointerException
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraft.world.World.func_72964_e(World.java:528)
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraft.world.World.func_72805_g(World.java:617)
2013-06-16 22:17:45 [INFO] [STDERR] at biomesoplenty.blocks.BlockBOPFlower.func_71854_d(BlockBOPFlower.java:271)
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraftforge.common.ForgeHooks.plantGrass(ForgeHooks.java:62)
2013-06-16 22:17:45 [INFO] [STDERR] at gregtechmod.GT_Mod.postload(GT_Mod.java:3364)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:494)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.post(EventBus.java:267)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:186)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:166)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
2013-06-16 22:17:45 [INFO] [STDERR] at com.google.common.eventbus.EventBus.post(EventBus.java:267)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:103)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.Loader.initializeMods(Loader.java:696)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.server.FMLServerHandler.finishServerLoading(FMLServerHandler.java:93)
2013-06-16 22:17:45 [INFO] [STDERR] at cpw.mods.fml.common.FMLCommonHandler.onServerStarted(FMLCommonHandler.java:355)
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:141)
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:431)
2013-06-16 22:17:45 [INFO] [STDERR] at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-06-16 22:17:45 [INFO] [STDERR] Minor Bug: Wasn't able to simulate the planting of a Flower with Bonemeal, to add Extractor Recipe for Dye:

Frankly, if it's Gregtech, then we couldn't care less. It fucks with so many other mods that it wouldn't surprise me if it fucks with BOP too. His problem, not ours.