Unable to create Sub-Tasks if the default assignee is a deactivated user

Issue Summary

The customer was unable to create Sub-Tasks due to the default assignee (the Project Leader) was deactivated. There is no error message of any type.

Steps to Reproduce

1. Deactivate the default assignee
2. Try to create a Sub-Task

Expected Results

An error message explaining why you were unable to create the Sub-Task

Actual Results

A page refresh with the Sub-Task not created.

Workaround

Activate the deactivated user or change the default assignee