title: Swift unwrapping optional giving error with data
body: <p>My iOS application fetch data from server and render some images from that data using 'Kingfisher', the problem is whenever i add or replace new image on server application crashes due to this code below with error :
  "<strong><em>fatal error: unexpectedly found nil while unwrapping an Optional value</em></strong>"</p>
  
  <pre><code>let prfix:String = "MY_PREFIX_URL /\(org.image)"
  celImage.kf_setImageWithURL((NSURL(string: prfix))!)
  </code></pre>
  
  <p>Please check the sceeenshot for more detail<a href="https://i.stack.imgur.com/mUoGV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mUoGV.jpg" alt="error screenshot"></a> 
  I don't undestand why I am getting this error, because you can see <code>prfix</code> got a value.</p>