When wrong files have been uploaded in chef (yes, i know, i'm really wrong). Like vim undofiles, chef-webui can't parse them and error is not catched, then the only result we see is:
undefined method `empty?' for nil:NilClass
See screenshot here: http://d.pr/i/zV3b
I've attached a pull request here : https://github.com/opscode/chef/pull/331
which solves the problem for me.