Hello everyone, I am a newbie to umbraco v8. I am trying to create an article list page and In that i am trying to display finite number of articles when the home page loads but I am not really understanding what does this error mean?

Error: Object reference not set to an instance of an object.

here is the code:

public ArticleResultSet GetLatestArticles(IPublishedContent currentContentItem, HttpRequestBase request)
    {
        var siteRoot = currentContentItem.Root();
        var articleList = GetArticleListPage(siteRoot);
        var articles = articleList.Descendants().Where(x => x.ContentType.Alias == "article" && x.IsVisible()).OrderByDescending(y => y.Value<DateTime>("articleDate"));
        var isArticleListPage = articleList.Id == currentContentItem.Id;
        var fallbackPageSize = isArticleListPage ? 10 : 3;

        var pageNumber = QueryStringHelper.GetIntFromQueryString(request, "page", 1);
        var pageSize = QueryStringHelper.GetIntFromQueryString(request, "size", fallbackPageSize);

        var pageOfArticles = articles.Skip((pageNumber - 1) * pageSize).Take(pageSize);

        var totalItemCount = articles.Count();
        var pageCount = totalItemCount > 0 ? Math.Ceiling((double)totalItemCount / pageSize) : 1;

        var resultSet = new ArticleResultSet() { 
            PageCount = pageCount, 
            PageNumber = pageNumber, 
            PageSize = pageSize, 
            Results = pageOfArticles, 
            IsArticleListPage = isArticleListPage,
            Url = articleList.Url
        };
        return resultSet;
    }

can someone please help me?