I've created a world using the new customized worlds system, and when I quit it, selected it and clicked the re-create, I went to the "More world settings", and removed the seed of my world. Then, while I still had the seed box selected, I clicked the "Customized" option. Then, the game just crashed. I don't know if it's because of the re-create option, the world I've just created or anything like that, but I know it isn't just a random thing, because I tried it a few times, and the same thing happens. I'm pretty sure it's because the new customized world system, but I have the crash report file, so you can look at that and fix it!