Fix NullRefException when cancelling the server.

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -170,6 +170,11 @@ namespace MQTTnet.Server
         {
             try
             {
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return;
+                }
+
                 var enqueuedApplicationMessage = _messageQueue.Take(cancellationToken);
 
                 var sender = enqueuedApplicationMessage.Sender;

Tests/MQTTnet.Core.Tests/MqttFactoryTests.cs

@@ -2,8 +2,6 @@
 using MQTTnet.Diagnostics;
 using MQTTnet.Extensions.ManagedClient;
 using System;
-using System.Collections.Generic;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;