Fix error caused by unknown nodes (#27)

gate_functions.lua

@@ -114,7 +114,8 @@ local rotate_pos_displaced = function(pos, origin, axis, direction)
 end
 
 local get_buildable_to = function(pos)
-	return minetest.registered_nodes[minetest.get_node(pos).name].buildable_to
+	local def = minetest.registered_nodes[minetest.get_node(pos).name]
+	return def and def.buildable_to
 end
 
 
@@ -290,9 +291,7 @@ local get_door_layout = function(pos, facedir, player)
 				if not vector.equals(door_node.pos, origin) then
 					-- There's no obstruction if the node is literally located along the rotation axis
 					local newpos = rotate_pos_displaced(door_node.pos, origin, axis, direction)
-					local newnode = minetest.get_node(newpos)
-					local newdef = minetest.registered_nodes[newnode.name]
-					if not newdef.buildable_to then
+					if get_buildable_to(newpos) then
 						-- check if the destination node is free.
 						door.swings[direction] = false
 						break