Got something like this when trying to install Mekanism with Gregtech.
Can someone make them work together or give me link to versions that work with each other?

---- Minecraft Crash Report ----
// Don't do that.

Time: 09.10.14 22:38
Description: Initializing game

java.lang.NullPointerException: Initializing game
at codechicken.multipart.MultiPartRegistry$$anonfun$registerParts$2.apply(MultiPartRegistry.scala:86)
at codechicken.multipart.MultiPartRegistry$$anonfun$registerParts$2.apply(MultiPartRegistry.scala:82)
at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:35)
at codechicken.multipart.MultiPartRegistry$.registerParts(MultiPartRegistry.scala:82)
at codechicken.multipart.MultiPartRegistry$.registerParts(MultiPartRegistry.scala:67)
at codechicken.multipart.MultiPartRegistry.registerParts(MultiPartRegistry.scala)
at codechicken.multipart.minecraft.Content.init(Content.java:30)
at codechicken.multipart.minecraft.MinecraftMultipartMod.preInit(MinecraftMultipartMod.java:19)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:513)
at sun.reflect.GeneratedMethodAccessor3.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:208)
at sun.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.creativemd.minecraftloader.MinecraftDummyContainer.callMethod(MinecraftDummyContainer.java:209)
at com.creativemd.minecraftloader.MinecraftDummyContainer.onModEvent(MinecraftDummyContainer.java:201)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74)
at com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:322)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304)
at com.google.common.eventbus.EventBus.post(EventBus.java:275)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:118)
at cpw.mods.fml.common.Loader.preinitializeMods(Loader.java:513)
at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:239)
at com.creativemd.minecraftloader.ClientHelper.startGame(ClientHelper.java:260)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:867)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:134)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

Looks like an FMP conflict, post it on their tracker.