title: fatal error: unexpectedly found nil while unwrapping an Optional value (cellForRowAtIndexPath) (Xib Custom Cell Swift)
body: <p>I get the error stated in the topic for unknown reason as the cell I reference(in DownloadProfilePicture) is visible. Basically, I am trying to replicate/adapt what is done in
  <a href="https://developer.apple.com/library/ios/samplecode/LazyTableImages/Introduction/Intro.html" rel="nofollow">LazyTableViewImages</a></p>
  
  <pre><code>override func viewDidLoad() {
           super.viewDidLoad()
           self.tableView.registerClass(topCell.self, forCellReuseIdentifier: "topCell")
           self.tableView.registerClass(contentCell.self, forCellReuseIdentifier: "contentCell")
           tableView.registerNib(UINib(nibName: "topCell", bundle: nil), forCellReuseIdentifier: "topCell")
           tableView.registerNib(UINib(nibName: "contentCell", bundle: nil), forCellReuseIdentifier: "contentCell")
           self.tableView.delegate = self
           self.tableView.dataSource = self
  }
  
  
  func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -&gt; UITableViewCell {
      let nodeCount = arrayOfPosts.count;
      let cell = UITableViewCell()
       tableView.dequeueReusableCellWithIdentifier("topCell", forIndexPath: indexPath) as! topCell
          if (nodeCount &gt; 0)
          {
              if (indexPath.row % 2 == 0){
                  // Set up the cell representing the app
                  var cell = tableView.dequeueReusableCellWithIdentifier("topCell", forIndexPath: indexPath) as! topCell
                  let post = arrayOfPosts[indexPath.row]
                  cell.userName.text = post.userName
                  cell.timeSincePosted.text = post.timeSincePosted
                  // Only load cached images; defer new downloads until scrolling ends
                  if (post.profileImage == nil)
                  {
                      if (!tableView.dragging &amp;&amp; !tableView.decelerating)
                      {
                          downloadProfileImage(post, indexPath: indexPath)
                          return cell
                      }
                      // if a download is deferred or in progress, return a placeholder image
                      cell.profileImage.image = UIImage(named: "titanic.jpg")
                                         }
                  else
                  {
                         cell.profileImage.image = post.profileImage
                  }
                  return cell
              }
  
  func downloadProfileImage(post: Post, indexPath: NSIndexPath){
          println(self.tableView.indexPathsForVisibleRows())
          println(indexPath)
          let cell:topCell = self.tableView.cellForRowAtIndexPath(indexPath) as! topCell
          cell.profileImage.image = UIImage(named: "img1")
      }
  </code></pre>
  
  <p>visiblePaths:</p>
  
  <pre><code> (
      "&lt;NSIndexPath: 0x7888f7b0&gt; {length = 2, path = 0 - 0}",
      "&lt;NSIndexPath: 0x788a4d60&gt; {length = 2, path = 0 - 1}"
  )
  </code></pre>
  
  <p>cell:</p>
  
  <pre><code>&lt;NSIndexPath: 0x788a3c80&gt; {length = 2, path = 0 - 1}
  </code></pre>