How to simulate a simple forum

The following code simulates a simple forum inside a wiki.  Use the "Add Comment" (at the bottom of the page) to reply.

<p>{{wiki.create("Create New Topic",page.path,_,true,"Put Your Title Here")}}</p>
<table cellspacing="0" cellpadding="4" border="1" block="var topics = [];
    foreach (var p in page.subpages) {
    var originator = wiki.getuser(web.list(p.api .. '/revisions?revision=1', '/page/user.author/username')[0]);
    var lastAuthor = originator;
    var lastDate = date.format(p.date, 's');
    if (#p.comments != 0) {
    var lastComment = list.reverse(p.comments)[0];
    if (date.isafter(lastComment.date,p.date)){
    let lastAuthor = lastComment.author;
    let lastDate =date.format(lastComment.date, 's');
    }
    }
    let topics ..= [ { page:p, originator:originator, lastAuthor:lastAuthor, lastDate:lastDate } ];
    }
    let topics = list.sort(topics, 'lastDate', true);" class="table">
    <tbody>
        <tr>
            <th valign="top" style="width: 70%;">Topic/Starter</th>
            <th valign="top" style="width: 15%;">Last Post</th>
            <th valign="top" style="width: 7%;">#Views</th>
            <th valign="top" style="width: 7%;">#Replies</th>
        </tr>
        <tr foreach="var t in topics" class="{{__count % 2 == 0 ? 'bg1' : 'bg2'}}">
            <td valign="top"><font style="font-size: 17px;"><strong>{{web.link(t.page.uri, t.page.title)}}</strong></font> <br />
            {{web.link(t.originator.uri, t.originator.name); }}</td>
            <td valign="top">{{date.format(t.lastDate,'yyyy-MM-d hh:mm');}} <br />
            by {{web.link(t.lastAuthor.uri, t.lastAuthor.name)}}</td>
            <td valign="top">{{t.page.viewcount}}</td>
            <td valign="top">{{#t.page.comments}}</td>
        </tr>
    </tbody>
</table>

Callstack:
    at User:Craigsivils/How_to_simulate_a_simple_forum
    at wiki.getuser

System.NullReferenceException: Object reference not set to an instance of an object
  at MindTouch.Deki.DekiWikiService.MakeUserObject (MindTouch.Deki.Data.UserBE user) [0x00000] in <filename unknown>:0 
  at MindTouch.Deki.DekiWikiService.WikiGetUser (System.Object user) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0