title: How to get nil if something can't be evaluated?
body: <p>I am using NSExpression to solve strings from user input. 
  Here is the code</p>
  
  <pre><code>var userInput = "5+"
  var solveUserInput = NSExpression(format: "\(userInput)")
  </code></pre>
  
  <p>It will crash my program because it can't solve it, but what I want to do is to return nil if the string is unsolvable by NSExpression. Is there any way to do this?</p>