Akka.Cluster and Akka.Cluster.Sharding: should throw human-friendly exception when accessing cluster / sharding plugins when clustering is not running

[Aaronontheweb]

Version Information
Version of Akka.NET? v1.4.43
Which Akka.NET Modules? Akka.Cluster, Akka.Cluster.Sharding

Describe the bug

When attempting to access the Cluster plugin when akka.actor.provider is not set to cluster:

var cluster = Cluster.Get(sys); // throws here
cluster.Join(cluster.SelfAddress);

// force cluster up

AwaitCondition(() => cluster.Members.Count(m => m.Status == Up) == 0);

Or the ClusterSharding plugin:

 shardingProxy = ClusterSharding // throws here
            .Get(Context.System)
            .Start(
                nameof(EntityActor),
                s => Props.Create(() => new EntityActor(s)),
                ClusterShardingSettings.Create(Context.System),
                new ShardingExtractor()
            );

The following exception is thrown:

resulting exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Akka.Cluster.ClusterSettings..ctor(Config config, String systemName)
   at Akka.Cluster.Cluster..ctor(ActorSystemImpl system)
   at Akka.Cluster.ClusterExtension.CreateExtension(ExtendedActorSystem system)
   at Akka.Actor.ExtensionIdProvider`1.Akka.Actor.IExtensionId.CreateExtension(ExtendedActorSystem system)
   at Akka.Actor.Internal.ActorSystemImpl.<>c__DisplayClass61_0.<RegisterExtension>b__1()

Expected behavior

It would be better if a less ambiguous error message was raised - i.e. an AkkaConfigurationException warning uses that their akka.cluster.provider or akka.cluster configuration sections might be missing / misconfigured.

Actual behavior

A very ambiguous NRE that doesn't help new Akka.Cluster users understand how to fix their problem.

Additional context

Doesn't ClusterSharding also throw a weird error message if we aren't currently part of a cluster? Shouldn't we make that clearer as well?