Replace TryGetValue() with GetValue()

The `TopicReferenceDictionary` had a `TryGetValue()` method, since it implemented `IDictionary<>`. The `KeyedCollection<>` used by the new underlying `TrackedCollection` doesn't implement this. We could add one, but it doesn't add significantly to the capabilities of `GetValue()`, which wil check to see if a value exists, and if not return an optional default value. As such, the call to `TryGetValue()` is replaced with a call to `GetValue()`.

(I refactored this a bit since another condition relies on the variable, and thus it needs to be defined in the parent scope. This is done when placing `TryGetValue()` instead a condition's _expression_, but wouldn't be done if I declared the variable within the _body_ of the condition.)

Author: JeremyCaney

OnTopic/Mapping/TopicMappingService.cs

@@ -281,6 +281,7 @@ private async Task SetPropertyAsync(
       \-----------------------------------------------------------------------------------------------------------------------*/
       var configuration         = new PropertyConfiguration(property, attributePrefix);
       var topicReferenceId      = source.Attributes.GetInteger($"{configuration.AttributeKey}Id", 0);
+      var topicReference        = source.References.GetValue(configuration.AttributeKey);
 
       if (topicReferenceId == 0 && configuration.AttributeKey.EndsWith("Id", StringComparison.OrdinalIgnoreCase)) {
         topicReferenceId        = source.Attributes.GetInteger(configuration.AttributeKey, 0);
@@ -314,7 +315,7 @@ private async Task SetPropertyAsync(
         }
       }
       else if (
-        source.References.TryGetValue(configuration.AttributeKey, out var topicReference) &&
+        topicReference is not null &&
         relationships.HasFlag(Relationships.References)
       ) {
         await SetTopicReferenceAsync(topicReference, target, configuration, cache).ConfigureAwait(false);