NullReferenceException in WorkItemDispatcher #73

I'm seeing the following exception stack being logged in vnext:

Exception: System.NullReferenceException : Object reference not set to an instance of an object.
at DurableTask.WorkItemDispatcher`1.d__37.MoveNext()

Looking at the tracing around it, it seems to be following this trace pattern:

1. TaskActivityDispatcher-997e1811751b4af7afa9705f6fa20c5b-0: Exception while processing workItem fc68b8e9ad0844e18f59bc71c817c2b9
   Exception: Microsoft.ServiceBus.Messaging.MessagingException : The service was unable to process the request
2. Backing off after exception by at least 10 until 5 successful operations
3. Abandoning message fc68b8e9ad0844e18f59bc71c817c2b9

I believe the issue lies within the ServiceBusOrchestrationService.cs -> AbandonTaskActivityWorkItemAsync method.

return message?.AbandonAsync();

If the message is null, it'd return null as a Task, which cannot be awaited on.