Fall back to content types of parents

The `ContentTypeListViewComponent` displays a list of content types available to be created under the current topic. It bases this list on the current topic's content type. This is confusing when creating a new topic, however, since the current topic hasn't yet been saved. If the user were to quit creating the current topic, and instead choose a new topic of a different content type, it would be operating against the parent context. As such, if the `actualTopic` can't be found—because it's a new topic!—it should fall back to the parent topic instead.

This additionally fixes a bug when creating a `Container`, since in this scenario it adopts the `PermittedContentTypes` from the current topic, instead of from the `ContentTypeDescriptor`, thus allowing the same `Container` class to target collections of different content types. This previously resulted in a null reference exception, however, since `actualTopic` was `null` in this scenario.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Components/ContentTypeListViewComponent.cs

@@ -100,13 +100,14 @@ public IViewComponentResult Invoke(
       | Get content type
       >-------------------------------------------------------------------------------------------------------------------------
       | If the database is uninitialized, the content type won't be found. In that case, return an empty view, which will
-      | effectively hide the component.
+      | effectively hide the component. If the topic cannot be found, assume it is a new topic and attempt to load the parent
+      | for context.
       \-----------------------------------------------------------------------------------------------------------------------*/
       var contentTypes          = _topicRepository.GetContentTypeDescriptors();
-      var actualTopic           = _topicRepository.Load(currentTopic.Id);
+      var actualTopic           = _topicRepository.Load(currentTopic.Id)?? _topicRepository.Load(currentTopic.Parent.Id);
       var actualContentType     = contentTypes.GetTopic(currentTopic.ContentType);
 
-      if (actualContentType is null) {
+      if (actualContentType is null || actualTopic is null) {
         return View(viewModel);
       }