When rejecting a "request to unpublish", the NPE error gets thrown

Reproduce:

1. Start a fresh installed Magnolia instance with demo data
2. Login with superuser
3. Go to page /travel/tour-finder (which has already be published)
4. Press unpublish -> in popup dialog press OK
5. In Pulse Go to the "Unpublish request" -> "Assign to me"
6. Press "Reject" -> in the dialog press "Save changes"
7. In the log you see the following error ...

The null pointer exception occurs, because the parameter "taskResult" is not available in the parameter map of function info.magnolia.module.workflow.jbpm.workitem.handler.RejectNotificationWorkItemHandler.getBody(WorkItem)