Error creating subtask in JIRA - Unknown error creating subtask for issue ABC-1234 - null

A couple of things can cause the message:
Error creating subtask in JIRA - Unknown error creating subtask for issue ABC-1234 - null

1. The user logged in Crucible may not exist in JIRA, or does not have permissions to create issues in that JIRA project.
   Probable solution:
   1. Crucible could use this JIRA REST API command in order to check if the user exists in JIRA.
   2. Improve this JIRA REST API command in order to retrieve which permission scheme the JIRA project is using, each kind of permission of that permission scheme, and all users/groups/roles allowed on each kind of permission. This would allow checking (combined with the REST API command above) if the user logged in has the expected permissions on the JIRA project linked to the Crucible project in question.
   3. Display a friendly message to the customer, with the results of the above two tests and links to the JIRA instance's create new user page and to the project permission scheme page.
2. The Crucible review the user is trying to create JIRA issues from may not have a JIRA issue linked to it.
   Probable solution:
   1. Use this Crucible REST API to check if the Crucible review has a JIRA issue key linked to it. The information will be on the element <jiraIssueKey>.
   2. Display a friendly message to the customer, with the results of the above test and a link to the Crucible review in edit mode focusing in the Linked Issue field.
3. A 'Closed' JIRA ticket is linked to the Crucible review the user is trying to create JIRA sub-tasks from. This is impossible yet: CRUC-3956
   Probable solution:
   1. Use this Crucible REST API to retrieve which JIRA issue is linked to that Crucible review.
   2. With the JIRA issue key in hands, use this JIRA REST API command to retrieve the JIRA issue Status information and check if it's valid.
   3. Display a friendly message to the customer, with the results of the above test and a link to the JIRA issue in question so the customer can transition the JIRA issue to a valid state, or with a link to the JIRA's Create New Issue page. If a new JIRA issue is created, this new JIRA issue should be set in the Crucible review's Linked Issue field.
4. The JIRA issue has mandatory custom fields that must be filled at the moment of issue creation. As Crucible is not aware of these JIRA custom fields, this can be causing the error. This bug was already reported: CRUC-2667
   Probable solution:
   1. Use this JIRA REST API command to retrieve the issue fields of the JIRA issue linked to that Crucible review and check whether any of these fields is a custom required field.
   2. Display a friendly message to the customer, with the results of the above test and a link to the correct Field Configuration in JIRA, where the customer is allowed to change the required custom fields to optional.