Fix NullReferenceException when calling Ping (#1834)

* Check connection status before doing ping

* Fix broken reconnect sample

* Update ReleaseNotes.md

.github/workflows/ReleaseNotes.md

@@ -0,0 +1 @@
+* [Client] Fixed NullReferenceExeption when performing a Ping when the client is not connected (#1831).

Samples/Client/Client_Connection_Samples.cs

@@ -430,6 +430,9 @@ public static class Client_Connection_Samples
                         }
                     }
                 });
+
+            Console.WriteLine("Press <Enter> to exit");
+            Console.ReadLine();
         }
     }
 }

Source/MQTTnet/Client/MqttClient.cs

@@ -238,6 +238,11 @@ namespace MQTTnet.Client
 
         public async Task PingAsync(CancellationToken cancellationToken = default)
         {
+            cancellationToken.ThrowIfCancellationRequested();
+            
+            ThrowIfDisposed();
+            ThrowIfNotConnected();
+
             if (cancellationToken.CanBeCanceled)
             {
                 await Request<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
@@ -441,7 +446,7 @@ namespace MQTTnet.Client
 
                 if (receivedPacket is MqttConnAckPacket connAckPacket)
                 {
-                    result = MqttClientResultFactory.ConnectResult.Create(connAckPacket, _adapter.PacketFormatterAdapter.ProtocolVersion);
+                    result = MqttClientResultFactory.ConnectResult.Create(connAckPacket, channelAdapter.PacketFormatterAdapter.ProtocolVersion);
                 }
                 else if (receivedPacket is MqttAuthPacket)
                 {
@@ -513,7 +518,7 @@ namespace MQTTnet.Client
             using (var effectiveCancellationToken = CancellationTokenSource.CreateLinkedTokenSource(backgroundCancellationToken, cancellationToken))
             {
                 _logger.Verbose("Trying to connect with server '{0}'", Options.ChannelOptions);
-                await _adapter.ConnectAsync(effectiveCancellationToken.Token).ConfigureAwait(false);
+                await channelAdapter.ConnectAsync(effectiveCancellationToken.Token).ConfigureAwait(false);
                 _logger.Verbose("Connection with server established");
 
                 _publishPacketReceiverQueue?.Dispose();