Ensure content type descriptor initialization

With the introduction of the new `GetContentTypeDescriptors()` overload (7a5a243), we inadvertantly introduced the possibility of bypassing the main `GetContentTypeDescriptors()` logic entirely, thus preventing even checking to see if content types exist in the database before looking for them in the local cache.

In practice, this probably wouldn't hurt anything. But it could potentially introduce problems if working with partial or disconnected topic graphs, where the 'Configuration:ContentTypes` is not part of the in-memory topic graph.

To mitigate this, we now call `GetContentTypeDescriptors()` from the new overload. But we don't want to either a) call that multiple times, or b) introduce a circular loop. So, to address that risk, we now exclusively initialize the `_contentTypeDescriptors` cache from within `GetContentTypeDescriptors()` method, and always access the cache through that.

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -59,6 +59,11 @@ public virtual ContentTypeDescriptorCollection GetContentTypeDescriptors() {
       \-----------------------------------------------------------------------------------------------------------------------*/
       if (_contentTypeDescriptors == null) {
 
+        /*----------------------------------------------------------------------------------------------------------------------
+        | Initialize cache
+        \---------------------------------------------------------------------------------------------------------------------*/
+        _contentTypeDescriptors = new ContentTypeDescriptorCollection();
+
         /*----------------------------------------------------------------------------------------------------------------------
         | Load configuration data
         \---------------------------------------------------------------------------------------------------------------------*/
@@ -105,10 +110,16 @@ public virtual ContentTypeDescriptorCollection GetContentTypeDescriptors() {
     protected virtual ContentTypeDescriptorCollection GetContentTypeDescriptors(ContentTypeDescriptor contentTypeDescriptors) {
 
       /*------------------------------------------------------------------------------------------------------------------------
-      | Initialize the collection
+      | Initialize the collection from the repository
+      >-------------------------------------------------------------------------------------------------------------------------
+      | ### NOTE JJC2020519: We want to centralize the shared logic from the public GetContentTypeDescriptors() while still
+      | ensuring the cache is first initialized from the underlying data store if this method is called directly. But we don't
+      | want to repeatedly call the underlying data store if it's empty, nor do we want to create a circular loop if this is
+      | being called from GetContentTypeDescriptors(). This is handled by initializing the _contentTypeDescriptors cache in the
+      | GetContentTypeDescriptors() method as a way of tracking the initialization state.
       \-----------------------------------------------------------------------------------------------------------------------*/
       if (_contentTypeDescriptors == null) {
-        _contentTypeDescriptors = new ContentTypeDescriptorCollection();
+        _contentTypeDescriptors = GetContentTypeDescriptors();
       }
 
       /*------------------------------------------------------------------------------------------------------------------------
@@ -280,19 +291,21 @@ public virtual int Save([ValidatedNotNull, NotNull]Topic topic, bool isRecursive
       /*------------------------------------------------------------------------------------------------------------------------
       | Validate content type
       \-----------------------------------------------------------------------------------------------------------------------*/
-      _contentTypeDescriptors = GetContentTypeDescriptors();
-      if (!_contentTypeDescriptors.Contains(topic.ContentType)) {
+      var contentTypeDescriptors= GetContentTypeDescriptors();
+      var contentTypeDescriptor = contentTypeDescriptors.GetTopic(topic.ContentType);
+
+      if (contentTypeDescriptor == null) {
         throw new ArgumentException(
           $"The Content Type \"{topic.ContentType}\" referenced by \"{topic.Key}\" could not be found under " +
-          $"\"Configuration:ContentTypes\". There are currently {_contentTypeDescriptors.Count} ContentTypes in the Repository."
+          $"\"Configuration:ContentTypes\". There are currently {contentTypeDescriptors.Count} ContentTypes in the Repository."
         );
       }
 
       /*------------------------------------------------------------------------------------------------------------------------
       | Update content types collection, if appropriate
       \-----------------------------------------------------------------------------------------------------------------------*/
-      if (topic is ContentTypeDescriptor && !_contentTypeDescriptors.Contains(topic.Key)) {
-        _contentTypeDescriptors.Add((ContentTypeDescriptor)topic);
+      if (topic is ContentTypeDescriptor && !contentTypeDescriptors.Contains(topic.Key)) {
+        contentTypeDescriptors.Add((ContentTypeDescriptor)topic);
       }
 
       /*------------------------------------------------------------------------------------------------------------------------