Patch for issue #3.

This doesn't fix the underlying issue of EnchantmentRevealer acting
weird when used on servers with radically different enchanting systems,
(I'm not sure there is a general fix for that), but it should stop
the crashing at least.

Author: d0sboots

java/io/github/d0sboots/enchantmentrevealer/ContainerEnchantmentWrapper.java

@@ -51,7 +51,7 @@ public ContainerEnchantmentWrapper(
     @Override
     public boolean enchantItem(EntityPlayer playerIn, int id) {
         boolean val = super.enchantItem(playerIn, id);
-        if (val) {
+        if (val && lastObservation != null) {
             Observation observation = new Observation();
             System.arraycopy(lastObservation.levels, 0, observation.levels, 0, 3);
             observation.truncatedSeed = (short) id;