Issue Summary

When trying to add a post function to a workflow transition, specifically the 'Update Issue Custom Field' function, an 'Error type: 500 - Internal server error' message is displayed if no parameters are set before clicking 'Add'. Instead of a 500 - Internal server error message, the system should prompt the user to complete the required fields.

Steps to Reproduce

1. In the workflow diagram, select any workflow transition
2. Access the 'Post Functions' screen
3. Click 'Add post function'
4. Choose 'Update Issue Custom Field' and click 'Add'
5. On the 'Add Parameters To Function' screen, don't make any changes, but click 'Add'

Expected Results

The system should prompt the user to complete the required fields.

Actual Results

An 'Error type: 500 - Internal server error' message is displayed.

Workaround

When adding the post function, make sure to select a field from the Issue Custom Field dropdown. Then click the Add button.