Pages data type selection

The null reference probably is hit because you have something like 'WHERE LEFNODEID = {%CurrentNodeID%}' which is fine but the CURRENT NODEID is unknown on NEW pages. As it's not yet inserted.

Could you share that simple query you mention?

here are the pieces of our selection code:

    //Add the necessary columns
    private string[] _resourceArticleColumns = new string[]
    {
        "ArticleName", "ArticleTitle", "ArticleTeaser", "RelatedArticlesList", "ArticleContent", "ArticleHeroImage", "ArticleTeaserImage", "NodeAliasPath", "ShowEmailForm", "CtaPath"
    };

     //Map the exported page type to the DTO object
     private Func<ResourceArticle, ResourceArticlePageDto> ResourceArticleSelect => resourceArticle => new ResourceArticlePageDto()
    {
        Header = resourceArticle.ArticleTitle,
        PageContent = resourceArticle.ArticleContent,
        HeroImage = resourceArticle.Fields.ArticleHeroImage,
        TeaserImage = resourceArticle.Fields.ArticleTeaserImage,
        TeaserText = resourceArticle.ArticleTeaser,
        NodeAliasPath = resourceArticle.NodeAliasPath,
        ShowEmailForm = resourceArticle.ShowEmailForm,
        SidebarCta = (!string.IsNullOrEmpty(resourceArticle.CtaPath) && resourceArticle.CtaPath != null) ? GetGlobalCtaByNodeGuid(resourceArticle.CtaPath) : null,
        **RelatedArticles = resourceArticle.Fields.RelatedArticlesList**
    }; 

    //Run the document query using an interface
    public ResourceArticlePageDto GetArticle(string nodeAliasPath)
    {
        return DocumentQueryService.GetDocument<ResourceArticle>(nodeAliasPath, true)
            .AddColumns(_resourceArticleColumns)
            .Select(ResourceArticleSelect)
            .FirstOrDefault();
    }

    //Document query service that gets the page type
     public DocumentQuery<TDocument> GetDocuments<TDocument>(bool loadBaseDtoFields = false) where TDocument : TreeNode, new()
    {
        var query = DocumentHelper.GetDocuments<TDocument>();

        // Evaluates whether preview mode is enabled
        if (SiteContext.IsPreviewEnabled)
        {
            if (loadBaseDtoFields)
                _coreColumns.Concat(new[] { "HideChildrenFromNavigation", "HideFromNavigation" });

            // Loads the latest version of pages as preview mode is enabled
            query = query
                .Columns(_coreColumns.Concat(new[] { "NodeSiteID" })) // Sets initial columns returned for optimization.
                                                                      //Adds 'NoteSiteD' column required for the Preview mode.
                .OnSite(SiteContext.SiteName) // There could be more sites with matching page
                .LatestVersion()
                .Published(false)
                .Culture(SiteContext.PreviewCulture);
        }

        if (!SiteContext.IsPreviewEnabled)
        {
            // Loads the published version of pages
            query = query
                .Columns(_coreColumns) // Sets initial columns returned for optimization.
                .OnSite(SiteContext.SiteName)
                .Published()
                .PublishedVersion()
                .Culture(SiteContext.CurrentSiteCulture);
        }

        return query;
    }

The bold line is where I am seeing the null reference error, which is actually happening in the generated code from Kentico at this point:

            /// <summary>
        /// Related Articles.
        /// </summary>
        public IEnumerable<TreeNode> RelatedArticlesList
        {
            get
            {
                return mInstance.GetRelatedDocuments("RelatedArticlesList");
            }
        }

RelatedArticlesList is an IEnumerable