Hi, did you install an extension? or modified the checkout files in your theme?, the demo is working fine.
Try to comment the checkout files, in: appdesignfrontenddefaulthellominitemplatecheckout
(like this __checkout), and if this works then do the same with the files inside this folder, so you will know wich file is causing this error. Doing this Magento will take the checkout files from his base, the same files that uses with the default theme.