The customizer is not yet 1.13 compatible.
This app is still being worked on.
It'll create a texture pack, but it won't be a very pleasant user experience. I've got a long list of things I want to improve, but for now, you can at least start using it and building your own custom version of Wolfhound while I keep making this better.
If you want to let me know about any problems or improvements you'd like to see, come chat with me on the Wolfhound Minecraft forum thread.
And if you're wanting to report a problem, please tell me what you were doing when it broke and let me know what browser you're using. That'll help a lot. Thanks!