fix crafting recipe detection for wood nodes

elepower_machines/crafting.lua

@@ -244,12 +244,14 @@ minetest.after(0.2, function ()
 	local assoc = {}
 	for _,wood in ipairs(wood_nodes) do
 		local recipes = minetest.get_all_craft_recipes(wood)
+		if recipes then
 			for _, recipe in ipairs(recipes) do
 				if recipe.items and #recipe.items == 1 then
 					assoc[recipe.items[1]] = wood
 				end
 			end
 		end
+	end
 
 	-- Register sawmill craft
 	for tree, wood in pairs(assoc) do