Moved check on Options.ValidateFeatures below ThrowIf methods to prevent NullReferenceException (#1800)

* Moved check on Options.ValidateFeatures below ThrowIf methods to prevent NullReferenceException

* Add Unit Test

* Update ReleaseNotes.md

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>

.github/workflows/ReleaseNotes.md

@@ -1,4 +1,5 @@
 * [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
+* [Client] Fixed _NullReferenceException_ when performing several actions when not connected (#1800, thanks to @ramonsmits).
 * [Server] Fixed _NullReferenceException_ in retained messages management (#1762, thanks to @logicaloud).
 * [Server] Exposed new option which allows disabling packet fragmentation (#1753).
 * [Server] Expired sessions will no longer be used when a client connects (#1756).

Source/MQTTnet.Tests/Clients/MqttClient/MqttClient_Connection_Tests.cs

@@ -196,5 +196,27 @@ namespace MQTTnet.Tests.Clients.MqttClient
                 Assert.AreEqual(response.UserProperties[0].Value, "Value");
             }
         }
+
+        [TestMethod]
+        public async Task Throw_Proper_Exception_When_Not_Connected()
+        {
+            try
+            {
+                var mqttFactory = new MqttFactory();
+                using (var mqttClient = mqttFactory.CreateMqttClient())
+                {
+                    await mqttClient.SubscribeAsync("test", MqttQualityOfServiceLevel.AtLeastOnce);
+                }
+            }
+            catch (MqttCommunicationException exception)
+            {
+                if (exception.Message == "The client is not connected.")
+                {
+                    return;
+                }
+            }
+
+            Assert.Fail();
+        }
     }
 }

Source/MQTTnet/Client/MqttClient.cs

@@ -321,14 +321,14 @@ namespace MQTTnet.Client
                 MqttTopicValidator.ThrowIfInvalidSubscribe(topicFilter.Topic);
             }
 
+            ThrowIfDisposed();
+            ThrowIfNotConnected();
+            
             if (Options.ValidateFeatures)
             {
                 MqttClientSubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter.PacketFormatterAdapter.ProtocolVersion);
             }
 
-            ThrowIfDisposed();
-            ThrowIfNotConnected();
-
             var subscribePacket = MqttPacketFactories.Subscribe.Create(options);
             subscribePacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();