title: How to check if NSMutableSet contains an element without nil error
body: <p>I'm new to Swift (and iOS programming), and I'm struggling with properly finding whether certain data structures contain elements.
  More specifically, I have created an empty set into which I want to insert elements after further calculations:</p>
  
  <pre><code>var moviesInItinerary : NSMutableSet? = nil
  </code></pre>
  
  <p>Then, I want to proceed with the calculations only if the set is empty or if it doesn't contain a given element.
  I wrote this condition like this:</p>
  
  <pre><code>if((moviesInItinerary?.count == 0) || (!moviesInItinerary!.containsObject(movies[currentMovie.row].title)))
  </code></pre>
  
  <p>However, when I try to run this code, I'm getting this error:</p>
  
  <blockquote>
    <p>fatal error: unexpectedly found nil while unwrapping an Optional value</p>
  </blockquote>
  
  <p>What is the safe way to perform these checks?</p>