Having the same user as Reporter and Assignee leaves the issue not transitionable through Customer Portal

Problem definition

If the Reporter on a request is also the Assignee, the request cannot be transitioned via the Customer Portal.

Steps to reproduce the issue

1. Show the Resolve this issue transition on the customer portal as per this guide.
2. As an Agent, create a request and assign it to yourself.
3. Go to the Customer Portal and click on Resolve this issue.
4. A pop-up for comment (optional) will be displayed, click on Resolve this issue.

Expected result

The issue is resolved.

Actual result

An error pop up, the issue is not resolved. An NPE below can be seen in the atlassian-jira.log.

2017-08-24 14:28:13,133 http-nio-8082-exec-12 url:/rest/servicedesk/1/custo.../transition username:agent1  ERROR agent1 596x219709x9 17nivfx 207.61.145.4,10.0.0.7 /rest/servicedesk/1/customer/requests/transition [c.a.p.r.c.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service: null
java.lang.NullPointerException
	at com.atlassian.jira.workflow.OSWorkflowManager.getWorkflow(OSWorkflowManager.java:276)
	at com.atlassian.jira.workflow.OSWorkflowManager.getActionDescriptor(OSWorkflowManager.java:389)
...

Workaround

• Transition the issue via Service Desk agent issue view.
• Assign the issue to other Agents.