add nil check for digger when using crystal shovel

crystal.lua

@@ -171,7 +171,8 @@ local old_handle_node_drops = minetest.handle_node_drops
 function minetest.handle_node_drops(pos, drops, digger)
 
 	-- are we holding Crystal Shovel?
-	if digger:get_wielded_item():get_name() ~= "ethereal:shovel_crystal" then
+	if not digger
+	or digger:get_wielded_item():get_name() ~= "ethereal:shovel_crystal" then
 		return old_handle_node_drops(pos, drops, digger)
 	end