title: Converting to Swift 3 error, value of optional type not unwrapped
body: <p>This code written with Swift 2 worked but not with Swift 3. </p>
  
  <pre><code>ShoppingManager.getShoppingItemsForPage(currentPage, pageSize: 40) { (shopItems: [ShopItemModel]?, error: Error?) -&gt; () in
          if error == nil {
  
  ...
  
  self.shopItems.append(Array(shopItems[10..&lt;40]))
  </code></pre>
  
  <p>I get an error</p>
  
  <p><code>value of optional type [ShopItemModel] not unwrapped</code>.</p>
  
  <p>I have tried using <code>?</code> but it doesn't work.</p>
  
  <p>Here is my ShopItemModel</p>
  
  <pre><code>class ShopItemModel: NSObject {
      //RestKitMapping
      var itemID: String?
      var shopName: String?
  </code></pre>