Interesting, because I don't get that error.
You're also not the first person to mention that the GUIs are broken in 0.18.13, but I am unable to get the error.
I've also asked some people to look into it for me, and none of them could get the error either.
Are there perhaps any other mods installed that may be deleting default style?
A list of other mods that you're using, or information about what you're doing that causes the error would help me narrow it down to try and fix it.
