Null reference exception during unpublish with custom resolver

Question

In our Tridion implementation (2013 SP1), we have built a custom resolver.

The resolver code is pretty simple, if we publish a component, and that component is of schema X, then also add all components added in a componentlinkfield to the published items

 var publishedComponent = item as Component;
 if (publishedComponent != null)
 {
    if (publishedComponent.Schema.Id.ItemId.Equals(<TCMURI>.ItemId))
    {
        ItemFields contentFields = new ItemFields(publishedComponent.Content, publishedComponent.Schema);
        if (contentFields.Contains("articles"))
        {
              ComponentTemplate template = context.Session.GetObject(new TcmUri(<TCMURI>)) as ComponentTemplate;
              ComponentLinkField linkedArticles = contentFields["articles"] as ComponentLinkField;
              if (linkedArticles != null && linkedArticles.Values != null)
              {
                  foreach (var linkedArticle in linkedArticles.Values)
                  {
                      if (resolvedItems.FirstOrDefault(r => r.Item.Id == linkedArticle.Id) == null)
                      {
                             resolvedItems.Add(new ResolvedItem(linkedArticle, template));
                      }
                   }
              }
         }             
    }
}

This is working fine when publishing - the item gets published, and all linked components get published as well.

However, when we unpublish a component of that specific schema, the resolver also runs well, but we receive a null reference exception a bit after that

The exception we get is

Object reference not set to an instance of an object.
Component: Tridion.ContentManager.Publishing
Errorcode: 0
User: NT AUTHORITY\SYSTEM

StackTrace Information Details:
   at System.Object.GetType()
   at Tridion.ContentManager.Publishing.Transporting.DefaultComponentHandler.HandleResolvedItemForUnPublishing(ResolvedItem resolvedItem)
   at Tridion.ContentManager.Publishing.Transporting.TransportPackage.AddResolvedItemsForUnPublishing(ISet`1 resolvedItems)
   at Tridion.ContentManager.Publishing.Handling.DefaultPublishTransactionHandler.HandlePublishRequest(PublishTransaction publishTransaction)
   at Tridion.ContentManager.Publishing.Handling.DefaultPublishTransactionHandler.ProcessPublishTransaction(PublishTransaction publishTransaction)
   at Tridion.ContentManager.Publishing.Publisher.QueueMessageHandler.HandleMessage()

Has anyone seen this error before ? Is there something specific we should do in our custom resolver ?

Answer 1

After some more debugging, we found the problem.

Actually, there was a bug in our code which was fixed some time ago, but the resolver dll was not yet updated on the CM server.

The bug was in this line

ComponentTemplate template = context.Session.GetObject(new TcmUri(<TCMURI>)) as ComponentTemplate;

the tcm uri was not the uri of a component template, but of a publication. This resulted in the template object being null.

As a result, the code

resolvedItems.Add(new ResolvedItem(linkedArticle, template));

was adding a resolved item object with a specific component, and with a template set to null. This gave no problems when publishing, but apparently throws a null reference exception after the resolver has run when we are unpublishing.