I found a issue while working on Workflows. I observed the same issue on JIRA 4.4.5 and JIRA 5.2.5
Steps to Reproduce on JIRA 4.4.5..
1. Workflows -> Choose any workflow and click 'Create Draft' workflow
2. Choose any transition and Select 'Validators' -> 'Add' Validator
3. Select 'Fields Required' and click 'Add'
4. Without selecting any field from the available fields, click 'Add' button at the bottom of the screen
5. System Error