Since I don't know how to get past the wrench in copying a debug, here's an easy recipe to reproduce the error.  

1. Open up a fresh image of Cuis, and a fresh browser.
2. Add a dummy category and class.
3. Add a test method to return a string.
3. Add text to the string before accepting it.
4. Repeat 3 or 4 times then an error will appear: 'Instances of UndefinedObject are not indexable'
5. Clicking on the text pane will produce additional errors with the same message.

This appears to be semi-random.  I have had it happen without the repeats in step 4.
It sometimes goes away for a while.

To insure reproducability, I copied text each time from http://www.biblegateway.com/passage/?search=Matthew+1&version=KJV

In my newbie opinion, it looks like it must happen to people frequently.  It also happens in Workspace.