ticking error when RF hammer breaks half slab blocks?

[Algester]

---- Minecraft Crash Report ----
// I blame Dinnerbone.

Time: 5/12/16 11:33 PM
Description: Unexpected error

java.lang.NullPointerException: Unexpected error
at ExAstris.Item.ItemHammerRF.onBlockStartBreak(ItemHammerRF.java:68)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78751_a(PlayerControllerMP.java:96)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78759_c(PlayerControllerMP.java:247)
at net.minecraft.client.Minecraft.func_147115_a(Minecraft.java:1357)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1967)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:973)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:898)
at net.minecraft.client.main.Main.main(SourceFile:148)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
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 --
Stacktrace:
at ExAstris.Item.ItemHammerRF.onBlockStartBreak(ItemHammerRF.java:68)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78751_a(PlayerControllerMP.java:96)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_78759_c(PlayerControllerMP.java:247)
at net.minecraft.client.Minecraft.func_147115_a(Minecraft.java:1357)

here's what I think is the important crash log?
the ticking error happens when it's inside a autonomous activator