Fix.

Build/MQTTnet.nuspec

@@ -22,7 +22,7 @@
 * [Server] Added interceptor for unsubscriptions.
 * [MQTTnet.Server] Added interceptor for unsubscriptions.
 * [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1
-* [MqttApplicationMessage] Added GetUserProperty<T> convenience method (thanks to @PMExtra).
+* [Core] Added MqttApplicationMessage.GetUserProperty&lt;T&gt;() convenience method (thanks to @PMExtra).
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2019</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 

Source/MQTTnet/Extensions/UserPropertyExtension.cs

@@ -7,7 +7,7 @@ namespace MQTTnet.Extensions
     {
         public static string GetUserProperty(this MqttApplicationMessage message, string propertyName, StringComparison comparisonType = StringComparison.OrdinalIgnoreCase)
         {
-            return message.UserProperties.SingleOrDefault(up => up.Name.Equals(propertyName, comparisonType))?.Value;
+            return message?.UserProperties?.SingleOrDefault(up => up.Name.Equals(propertyName, comparisonType))?.Value;
         }
 
         public static T GetUserProperty<T>(this MqttApplicationMessage message, string propertyName, StringComparison comparisonType = StringComparison.OrdinalIgnoreCase)