public partial class NewBlog : EPiServer.UserControlBase
    {
        private readonly string _removeInvalidRegEx = @"(</?)(i|p|b|strong|em)[^>]*(>)|<[^>]*>";
        private readonly string _restoreValidTagsRegEx = @"&lt;(/?)(i|p|b|strong|em)&gt;";

        protected void PostComment_Click(object sender, System.EventArgs e)
        {
            // Make sure that the page, and particurlarly the captcha, is valid.
            if (Page.IsValid)
            {
                PageData newPost = DataFactory.Instance.GetDefaultPageData(CurrentPage.PageLink,
                                                                              BlogUtil.ItemPageTypeName);

                // comment is used by a PropertyXhtmlString, and XhtmlString are built to work with an editor. Hence, they expect encoded text.
                // Therefor we must html encode the content.
                string post =
                    Server.HtmlEncode(Regex.Replace(PostTextNew.Text, _removeInvalidRegEx, "$1$2$3", RegexOptions.IgnoreCase));
                
                // name will be saved in a PropertyString, and should not be encoded. PropertyString encodeds itself when it's rendered.
                string name = Regex.Replace(NewPostName.Text, _removeInvalidRegEx, "$1$2$3", RegexOptions.IgnoreCase);

                post = Regex.Replace(post, _restoreValidTagsRegEx, "<$1$2>", RegexOptions.IgnoreCase);
                name = Regex.Replace(name, _restoreValidTagsRegEx, "<$1$2>", RegexOptions.IgnoreCase);

                newPost.PageName = name;
                newPost["MainBody"] = post;
                newPost.VisibleInMenu = false;

                // To get the new comment into the list. If we don't do this the
                // StartPublish value will be greater than the one used for
                // the publishing-filter.
                newPost.StartPublish = DateTime.Now.AddMinutes(-1);

                DataFactory.Instance.Save(newPost, EPiServer.DataAccess.SaveAction.Publish, AccessLevel.NoAccess);

                PostTextNew.Text = String.Empty;
                NewPostName.Text = String.Empty;

              
            }
        }//End

        protected void AddUrl_Click(object sender, System.EventArgs e)
        {
            //btnAdd.Click += new EventHandler(this.AddUrl_Click);
            LinkItemCollection links = ((LinkItemCollection)CurrentPage["LinkCollectionProperty"]);
            //LinkName  -  LinkUrl
            
            LinkItem link = new LinkItem();
            link.Href = LinkUrl.Text;
            link.Text = LinkName.Text;
            link.Title = LinkName.Text;

            links.Add(link);

        }
    }
}

Getting the EM:

Object reference not set to an instance of an object.

Which line gets the error? My guess is that the links variable is null, and then you do Add on it. If the property has not been previously set on the page, its value will be null.

Mind also that properties are read-only unless you create a writeable clone of the parent page first.