MNTR: `NullRefeferencException` inside `Throttler`

[Aaronontheweb]

Version Information Version of Akka.NET? 1.5.39 Which Akka.NET Modules? Akka.Remote / Akka.Remote.TestKit

Describe the bug

Hit the following NRE while working on troubleshooting DistributedPubSubRestartSpecs:

[ERROR][03/23/2025 12:53:13.315Z][Thread 0006][akka://DistributedPubSubRestartSpec/system/transports/throttlermanager.$gremlin.tcp$1/throttler1] Object reference not set to an instance of an object.
Cause: System.NullReferenceException: Object reference not set to an instance of an object.
   at Akka.Actor.ActorRefBase.Equals(IActorRef other) in E:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorRef.cs:line 405
   at Akka.Actor.ActorCell.RemWatcher(IActorRef watchee, IActorRef watcher) in E:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorCell.DeathWatch.cs:line 267
   at Akka.Actor.ActorCell.SysMsgInvokeAll(EarliestFirstSystemMessageList messages, Int32 currentState) in E:\Repositories\olympus\akka.net\src\core\Akka\Actor\ActorCell.DefaultMessages.cs:line 288

Expected behavior

Either null shouldn't be passable here or the method should be able to handle it.

Actual behavior

NRE.

[Aaronontheweb]

Oof, even worse this is happening during Context.Watch