The button IDs map to the class/race DBC's, from what I remember. Or something similar. So when you have a button or class for that which doesn't exist in the DBFilesClient, the client crashes.
The Lua errors are quite clear. You are moaning about them but not reading them.
It tells you the line number and the error - a nil value means that it is trying to use something that doesn't exist. Look at that line of code and the code around it, you can use the "message" function to display debug information to the screen. Usually when you fix the error the rest of the code executes fine and it goes from completely non-functional to perfect instantly.
A tip would be to not rely on "reloadui" from console when doing changes. It does give you a quick look at what you changed but it seems to render it in a different order to when the char creation screen is first loaded up on a new instance of WoW. I had a lot of issues trying to figure out why it would only sometimes work until I realised this then quickly fixed the issues.
