6.x: Guard against null when removing ShutdownHandler

projects/RabbitMQ.Client/client/impl/ModelBase.cs

@@ -681,7 +681,10 @@ public void HandleBasicCancelOk(string consumerTag)
                 k.m_consumer = DefaultConsumer;
             }
 
-            ConsumerDispatcher.HandleBasicCancelOk(k.m_consumer, consumerTag);
+            if (!(k.m_consumer is null))
+            {
+                ConsumerDispatcher.HandleBasicCancelOk(k.m_consumer, consumerTag);
+            }
 
             k.HandleCommand(IncomingCommand.Empty); // release the continuation.
         }
@@ -1073,7 +1076,10 @@ public void BasicCancel(string consumerTag)
                 _consumers.Remove(consumerTag);
             }
 
-            ModelShutdown -= k.m_consumer.HandleModelShutdown;
+            if (!(k.m_consumer is null))
+            {
+                ModelShutdown -= k.m_consumer.HandleModelShutdown;
+            }
         }
 
         public void BasicCancelNoWait(string consumerTag)

projects/Unit/TestBasicCancelAnyConsumer.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Threading;
+using NUnit.Framework;
+using RabbitMQ.Client.Events;
+
+namespace RabbitMQ.Client.Unit
+{
+    [TestFixture]
+    public class TestBasicCancelAnyConsumer : IntegrationFixture
+    {
+        [Test]
+        public void TestBasicCancelWithQueueDelete()
+        {
+            // Arrange
+            Exception modelException = null;
+            Model.CallbackException += HandleCallbackException;
+            Model.QueueDeclare(TestContext.CurrentContext.Test.Name, false, false, false);
+            string consumerTag = Model.BasicConsume(TestContext.CurrentContext.Test.Name, false, new VoidConsumer());
+
+            // Act
+            WithTemporaryModel(m => m.QueueDelete(TestContext.CurrentContext.Test.Name));
+            TestDelegate act = () => Model.BasicCancel(consumerTag);
+
+            // Assert
+            Assert.DoesNotThrow(act);
+            SpinWait.SpinUntil(() => !(modelException is null), TimingFixture.TestTimeout);
+            Assert.IsNull(modelException, "The model has thrown an exception when cancelling a consumer after the queue has been deleted. This exception comes from the scheduling of CancelOk Action.");
+
+            // Cleanup
+            Model.CallbackException -= HandleCallbackException;
+
+            void HandleCallbackException(object sender, CallbackExceptionEventArgs e) => modelException = e.Exception;
+        }
+
+        private sealed class VoidConsumer : DefaultBasicConsumer{}
+    }
+}