dont allow publish before the client is started and adjusted testclient accordingly

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -49,8 +49,6 @@ namespace MQTTnet.Extensions.ManagedClient
 
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _logger = logger.CreateChildLogger(nameof(ManagedMqttClient));
-
-            Options = new ManagedMqttClientOptions();
         }
 
         public bool IsConnected => _mqttClient.IsConnected;
@@ -150,6 +148,7 @@ namespace MQTTnet.Extensions.ManagedClient
             ThrowIfDisposed();
 
             if (applicationMessage == null) throw new ArgumentNullException(nameof(applicationMessage));
+            if (Options == null) throw new InvalidOperationException("call StartAsync before publishing messages");
 
             MqttTopicValidator.ThrowIfInvalid(applicationMessage.ApplicationMessage.Topic);
 

Tests/MQTTnet.TestApp.NetCore/ManagedClientTest.cs

@@ -40,11 +40,11 @@ namespace MQTTnet.TestApp.NetCore
                     Console.WriteLine(">> RECEIVED: " + e.ApplicationMessage.Topic);
                 });
 
-                await managedClient.PublishAsync(builder => builder.WithTopic("Step").WithPayload("1"));
-                await managedClient.PublishAsync(builder => builder.WithTopic("Step").WithPayload("2").WithAtLeastOnceQoS());
-
                 await managedClient.StartAsync(options);
 
+                await managedClient.PublishAsync(builder => builder.WithTopic("Step").WithPayload("1"));
+                await managedClient.PublishAsync(builder => builder.WithTopic("Step").WithPayload("2").WithAtLeastOnceQoS());
+                
                 await managedClient.SubscribeAsync(new TopicFilter { Topic = "xyz", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce });
                 await managedClient.SubscribeAsync(new TopicFilter { Topic = "abc", QualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce });