Fix null ref exception in low level client.

Build/MQTTnet.nuspec

@@ -12,12 +12,13 @@
     <requireLicenseAcceptance>true</requireLicenseAcceptance>
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports v3.1.0, v3.1.1 and v5.0.0 of the MQTT protocol.</description>
     <releaseNotes>
-* [RcpClient] Adjusted some namespaces (BREAKING CHANGE!)
-* [AspNetCore] Adjusted some namespaces (BREAKING CHANGE!)
-* [Server] Adjusted some namespaces (BREAKING CHANGE!)
+* [LowLevelClient] Fixed a null reference exception when connecting to a not existing server (thanks to @SGStino).      
+* [RcpClient] Adjusted some namespaces (BREAKING CHANGE!).
+* [AspNetCore] Adjusted some namespaces (BREAKING CHANGE!).
+* [Server] Adjusted some namespaces (BREAKING CHANGE!).
 * [Server] Added state checks (throw if not started etc.) for most server APIs.
 * [Server] Exposed real X509Certificate2 (instead byte array) to TLS options (thanks to @borigas).
-* [Server] Fixed memory leak with TCP sockets (MqttServer is now Disposable!) (BREAKING CHANGE!)
+* [Server] Fixed memory leak with TCP sockets (MqttServer is now Disposable!) (BREAKING CHANGE!).
 * [Core] Fixed a null reference exception in the MqttTcpChannel with WriteAsync and ReadAsync.
 * [Core] Added server interceptor for undelivered messages (thanks to @cshark-inator).
 * [nuget] Added support for SourceLink (thanks to @JTOne123).

Source/MQTTnet/LowLevelClient/LowLevelMqttClient.cs

@@ -48,7 +48,7 @@ namespace MQTTnet.LowLevelClient
             }
             catch (Exception)
             {
-                _adapter.Dispose();
+                _adapter?.Dispose();
                 throw;
             }
 

Tests/MQTTnet.Core.Tests/LowLevelMqttClient_Tests.cs

@@ -8,6 +8,7 @@ using System.Collections.Generic;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Exceptions;
 
 namespace MQTTnet.Tests
 {
@@ -16,6 +17,18 @@ namespace MQTTnet.Tests
     {
         public TestContext TestContext { get; set; }
 
+        [TestMethod]
+        [ExpectedException(typeof(MqttCommunicationException))]
+        public async Task Connect_To_Not_Existing_Server()
+        {
+            var client = new MqttFactory().CreateLowLevelMqttClient();
+            var options = new MqttClientOptionsBuilder()
+                .WithTcpServer("localhost")
+                .Build();
+
+            await client.ConnectAsync(options, CancellationToken.None).ConfigureAwait(false);
+        }
+
         [TestMethod]
         public async Task Connect_And_Disconnect()
         {
@@ -78,7 +91,7 @@ namespace MQTTnet.Tests
 
         async Task<MqttConnAckPacket> Authenticate(ILowLevelMqttClient client)
         {
-            await client.SendAsync(new MqttConnectPacket()
+            await client.SendAsync(new MqttConnectPacket
             {
                 CleanSession = true,
                 ClientId = TestContext.TestName,