Check for ndef before registering Stairs+ nodes.

stairsplus/registrations.lua

@@ -35,6 +35,7 @@ local default_nodes = { -- Default stairs/slabs/panels/microblocks:
 for _, name in pairs(default_nodes) do
 	local nodename = "default:" .. name
 	local ndef = minetest.registered_nodes[nodename]
+	if ndef then
 		local groups = {}
 		for k, v in pairs(ndef.groups)
 			-- Ignore wood and stone groups to not make them usable in crafting:
@@ -54,5 +55,6 @@ for _, name in pairs(default_nodes) do
 			tiles = ndef.tiles,
 			sunlight_propagates = true,
 		})
+	end
 end