Nested Ask async await causes null-pointer exception in ActorTaskScheduler

[Danthar]

When nesting Ask statements as described in this gist: https://gist.github.com/Danthar/d1fe5581a6ba13ca6e30
A problem occurs causing the first Ask to never return. An exception from the ActorTaskScheduler seems to be the culprit.

[rogeralsing]

The issue have been identified to be at:
https://github.com/akkadotnet/akka.net/blob/dev/src/core/Akka/Dispatch/ActorTaskScheduler.cs#L56

Ambient state can be null in cases where e.g. a TaskCompletionSource is beeing completed from the outside, e.g. in a FutureActorRef while the awaiting code is under the ActorTaskScheduler control.

[rogeralsing]

PR merged