JIRA issue sub-tasks created however system throws error message

Though the action was successful, in a number of projects, customers are hitting into an exception error message (Exception occurred: java.lang.NullPointerException: null value), when creating sub-tasks or issues. Screen-shot of error message is attached.

Steps to Reproduce

• On any JIRA existing issue, try to create a sub-task, or
• Raise a JIRA issue/story with unassigned

Expected Results
Issues or sub-tasks should be raised just fine.

Actual Results
Error message (Exception occurred: java.lang.NullPointerException: null value) is being thrown from the system.
The sub-task is created successfully though you see the error message.

Current Workaround
Unmark the 'Security Level' field to be displayed on the issue create screen (refer screenshot below).

Dependancy:
Requires the 'Security Level' field to be displayed to trigger the behaviour