Null fix when handling transitions

monarch/transitions/gui.lua

@@ -79,8 +79,10 @@ function M.create(node)
 	
 	-- Forward on_message calls here
 	function instance.handle(message_id, message, sender)
+		if transitions[message_id] then
 			transitions[message_id](sender)
 		end
+	end
 	
 	-- Specify the transition function when this node is transitioned
 	-- to