Hardening MessageContext.FlushOutgoingMessags() against nulls. Closes GH-2006

Author: jeremydmiller

src/Wolverine/Runtime/MessageBus.cs

@@ -252,6 +252,8 @@ public ValueTask BroadcastToTopicAsync(string topicName, object message, Deliver
 
     internal async ValueTask PersistOrSendAsync(Envelope envelope)
     {
+        if (envelope is null) return; // Not sure how this would happen
+        
         if (envelope.Sender is null)
         {
             throw new InvalidOperationException("Envelope has not been routed");
@@ -310,7 +312,7 @@ internal async ValueTask PersistOrSendAsync(params Envelope[] outgoing)
             // the sender is currently latched
             var envelopes = outgoing.Where(isDurable).ToArray();
             foreach (var envelope in envelopes.Where(x =>
-                         x.Sender is { Latched: true } && x.Status == EnvelopeStatus.Outgoing))
+                         x is { Sender: { Latched: true }, Status: EnvelopeStatus.Outgoing }))
                 envelope.OwnerId = TransportConstants.AnyNode;
 
             await Transaction.PersistAsync(envelopes);

src/Wolverine/Runtime/MessageContext.cs

@@ -109,7 +109,6 @@ public async Task FlushOutgoingMessagesAsync()
                         $"This MessageContext does not allow multiple calls to {nameof(FlushOutgoingMessagesAsync)} because {nameof(MultiFlushMode)} = {MultiFlushMode}");
             }
         }
-
 
         await AssertAnyRequiredResponseWasGenerated();
 
@@ -120,6 +119,9 @@ public async Task FlushOutgoingMessagesAsync()
 
         foreach (var envelope in Outstanding)
         {
+            // https://github.com/JasperFx/wolverine/issues/2006
+            if (envelope == null) continue;
+            
             try
             {
                 if (envelope.IsScheduledForLater(DateTimeOffset.UtcNow))