Add a null check on transaction for Error (#2618)

Add a null check on transaction for Error'

For issue: #2553

Author: JeremyBessonElastic

src/Elastic.Apm/Model/Error.cs

@@ -2,7 +2,6 @@
 // Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -66,13 +65,16 @@ private void CheckAndCaptureBaggage(Transaction transaction)
 
 			//if context was not set prior we set it now to ensure we capture baggage for errors
 			//occuring during unsampled transactions
-			Context ??= transaction.Context.DeepCopy();
+			Context ??= transaction?.Context.DeepCopy();
 
 			foreach (var baggage in Activity.Current.Baggage)
 			{
 				if (!WildcardMatcher.IsAnyMatch(Configuration.BaggageToAttach, baggage.Key))
 					continue;
 
+				// The context is created only if there is a baggage value to insert.
+				Context ??= new Context();
+
 				var newKey = $"baggage.{baggage.Key}";
 				var labels = Context.InternalLabels.Value;
 				labels[newKey] = baggage.Value;