Added null conditional operator to sourceTopic

Added check for cases where `sourceTopic`  may be null (e.g., for legacy pages not associated with a Topic).

Author: ktrunkey

Ignia.Topics.Web.Mvc/Controllers/LayoutControllerBase{T}.cs

@@ -178,7 +178,7 @@ protected Topic GetNavigationRoot(Topic currentTopic, int fromRoot = 2, string d
     /// <param name="sourceTopic">The <see cref="Topic"/> to pull the values from.</param>
     private static int DistanceFromRoot(Topic sourceTopic) {
       var distance = 1;
-      while (sourceTopic.Parent != null) {
+      while (sourceTopic?.Parent != null) {
         sourceTopic = sourceTopic.Parent;
         distance++;
       }