Add a null check in case there is no current equipped item

src/main/java/tconstruct/library/tools/HarvestTool.java

@@ -289,7 +289,10 @@ public abstract class HarvestTool extends ToolCore
         }
 
         // callback to the tool the player uses. Called on both sides. This damages the tool n stuff.
-        player.getCurrentEquippedItem().func_150999_a(world, block, x, y, z, player);
+        ItemStack currentItem = player.getCurrentEquippedItem();
+        if(currentItem != null) {
+            currentItem.func_150999_a(world, block, x, y, z, player);
+        }
 
         // server sided handling
         if (!world.isRemote) {