I was mining normal stone and got this crash.

ava.lang.NullPointerException: Unexpected error
at com.ewyboy.hammertime.items.ItemBaseTool.func_179218_a(ItemBaseTool.java:55)
at net.minecraft.item.ItemStack.func_179548_a(ItemStack.java:376)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_187103_a(PlayerControllerMP.java:166)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_180511_b(PlayerControllerMP.java:248)
at PlayerControllerOF.func_180511_b(PlayerControllerOF.java:49)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_180512_c(PlayerControllerMP.java:334)
at PlayerControllerOF.func_180512_c(PlayerControllerOF.java:65)
at net.minecraft.client.Minecraft.func_147115_a(Minecraft.java:1478)
at net.minecraft.client.Minecraft.func_184117_aA(Minecraft.java:2266)
at net.minecraft.client.Minecraft.func_184118_az(Minecraft.java:2018)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1806)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1096)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:397)
at net.minecraft.client.main.Main.main(SourceFile:123)
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:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Client thread
Stacktrace:
at com.ewyboy.hammertime.items.ItemBaseTool.func_179218_a(ItemBaseTool.java:55)
at net.minecraft.item.ItemStack.func_179548_a(ItemStack.java:376)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_187103_a(PlayerControllerMP.java:166)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_180511_b(PlayerControllerMP.java:248)
at PlayerControllerOF.func_180511_b(PlayerControllerOF.java:49)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_180512_c(PlayerControllerMP.java:334)
at PlayerControllerOF.func_180512_c(PlayerControllerOF.java:65)
at net.minecraft.client.Minecraft.func_147115_a(Minecraft.java:1478)
at net.minecraft.client.Minecraft.func_184117_aA(Minecraft.java:2266)
at net.minecraft.client.Minecraft.func_184118_az(Minecraft.java:2018)

Thanks, gonna look into it.

Fixed in version 2.2.0