Estou criando um aplicativo no Swift, mas continuo recebendo um erro na minha classe TableViewController. Não consegui encontrar nenhuma maneira de corrigir isso e continuei recebendo este erro:

override func tableView(tableView: UITableView?, cellForRowAtIndexPath indexPath: NSIndexPath!) -> UITableViewCell!  {
        let cell : TextTableViewCell = tableView!.dequeueReusableCellWithIdentifier("Cell", forIndexPath : indexPath!) as TextTableViewCell

    let madan : PFObject = self.timeLineData.objectAtIndex(indexPath!.row) as PFObject

    cell.timestampLabel.alpha = 0
    cell.usernameLabel.alpha = 0
    cell.madanTextView.alpha = 0

    cell.madanTextView.text = madan.objectForKey("content") as String

    var dateFormatter : NSDateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "MM-dd-yyyy HH:mm"
    cell.timestampLabel.text = dateFormatter.stringFromDate(madan.createdAt)

    var findSender : PFQuery = PFUser.query()
    findSender.whereKey("objectId", equalTo: madan.objectForKey("sender").objectId)
    var i = 1
    findSender.findObjectsInBackgroundWithBlock {
        (objects: [AnyObject]!, error: NSError!) -> Void in
        if !error {
            let user : PFUser = (objects as NSArray).lastObject as PFUser // Error here

            cell.usernameLabel.text = user.username 

            UIView.animateWithDuration(0.5, animations: {

                cell.timestampLabel.alpha = 1
                cell.usernameLabel.alpha = 1
                cell.madanTextView.alpha = 1

            })
        }
    }    
    return cell
}

Não consigo encontrar nenhuma maneira de corrigir isso.