Projects feature: Application error when delayed blocks/pages are published

Description

Step to reproduce:

1. Select/create a project.
2. Make a new page and set its status to "Ready to publish".
3. Go to the project overview.
4. Under options, select "Schedule Items for Publish".
5. Set the publish date to today's date a couple of minutes into the future.
6. Let the "Publish Delayed Content Versions" job run.
7. Check "Event Viewer" (remember to update).

Expected: No error.
Actual: Error thrown "The process was terminated due to an unhandled exception."

Stack trace:
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ArgumentNullException
at EPiServer.Notification.NotificationUser.set_UserName(System.String)
at EPiServer.Cms.Shell.UI.Notifications.ProjectMessageNotifier+<CreateNotificationMessage>d__17.MoveNext()
at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.<ThrowAsync>b__6_1(System.Object)
at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
at System.Threading.ThreadPoolWorkQueue.Dispatch()