Account for potentially null result

Resolves `CS8602`. In the `ReverseTopicMappingService`, the `MapAsync()` method could, potentially, return a `null` object. In this case, it shouldn't be added to the `targetList`. This is an unexpected situation, but an easy condition to check for, while also satisfying code analysis.

Author: JeremyCaney

OnTopic/Mapping/Reverse/ReverseTopicMappingService.cs

@@ -608,7 +608,7 @@ TopicCollection           targetList
         var topicTask = await Task.WhenAny(taskQueue).ConfigureAwait(false);
         taskQueue.Remove(topicTask);
         var topic = await topicTask.ConfigureAwait(false);
-        if (!targetList.Contains(topic.Key)) {
+        if (topic != null && !targetList.Contains(topic.Key)) {
           targetList.Add(topic);
         }
       }