Broken view when adding a duplicate property to a XWikiCLass

When I attempt to add a property that already exists to an XWikiClass, the view breaks, and I receive the popup shown in the image

Steps to reproduce:

1. Go to /xwiki/bin/view/XWiki/XWikiClasses
2. Create a class named "test"
3. Go to /xwiki/bin/edit/XWiki/testClass?editor=class
4. Add a new property, e.g. one named 'test' and of any type
5. Observe that the property has been added
6. Add the same property with the same name ('test') and of any type

Expected results: A simple pop-up that explains that a property with that name already exists.
Actual results: A broken view that doesn't explain anything.

This bug could be really annoying when the class has a lot of properties because the user would not realize that the error was caused by a duplicate name.