[Critical] Fixed issue with DeriedTopic

Ensured that setting the `DerivedTopic` would not fail if the `DerivedTopic` wasn't already set.

Author: JeremyCaney

Ignia.Topics/Topic.cs

@@ -513,7 +513,7 @@ public Topic DerivedTopic {
           value != this,
           "A topic may not derive from itself."
         );
-        if (!_derivedTopic.Equals(value)) {
+        if (!_derivedTopic?.Equals(value)?? false) {
           return;
         }
         _derivedTopic = value;