Help adding mods

[Jams]

Since nobody else was responding to my other thread, I'm making a new one.

When trying to add Better Farming 2.3.1 Forge version to my Tekkit Classic client I'm getting this error.

java.lang.NullPointerException
    at mod_BuildCraftCore$EntityRenderIndex.hashCode(mod_BuildCraftCore.java:48)
    at java.util.HashMap.hash(Unknown Source)
    at java.util.HashMap.put(Unknown Source)
    at mod_BuildCraftEnergy.modsLoaded(mod_BuildCraftEnergy.java:43)
    at cpw.mods.fml.common.modloader.ModLoaderModContainer.postInit(ModLoaderModContainer.java:362)
    at cpw.mods.fml.common.Loader.postModInit(Loader.java:290)
    at cpw.mods.fml.common.Loader.initializeMods(Loader.java:629)
    at cpw.mods.fml.client.FMLClientHandler.onLoadComplete(FMLClientHandler.java:223)
    at net.minecraft.client.Minecraft.a(Minecraft.java:429)
    at net.minecraft.client.Minecraft.run(Minecraft.java:738)
    at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT c7cd1d3b ----------

Once this is resolved I'm going to try to add Twilight Forest after this.