When looking up a topic by RouteData, swallow InvalidKeyException

As route data comes from user-submitted requests, it's expected that some may contain invalid keys. From that perspective, an invalid key is no different than having an invalid URL. As such, we should return a null result, not bubble up an exception.

In practical terms, that means that calling e.g. `/Web/Invalid/Url` and calling `/Web/Invalid/~Url` will both end up returning a 404 error when processed by the `[ValidateTopic]` attribute. By contrast, previously, the former would return a 404 while the latter would return a 500 error due to the `InvalidKeyException`.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/TopicRepositoryExtensions.cs

@@ -79,7 +79,14 @@ RouteData routeData
       foreach (var searchPath in paths) {
         if (topic is not null) break;
         if (String.IsNullOrEmpty(searchPath)) continue;
-        topic = topicRepository.Load(searchPath);
+        try {
+          topic = topicRepository.Load(searchPath);
+        }
+        catch (InvalidKeyException) {
+          //As route data comes from user-submitted requests, it's expected that some may contain invalid keys. From this
+          //method's perspective, this is no different than having an invalid URL. As such, we should return a null result, not
+          //bubble up an exception.
+        }
       }
 
       return topic;