title: How to make NSURL variable is nil when parsed from null String?
body: <p>I'm a newbie in iOS development. Sometimes my API server returns a <code>nil</code> value. This is raising an error when I try to create a <code>NSURL</code> instance.</p>
  
  <p>Here is the condition:</p>
  
  <pre><code>NSString *tmpURL = nil;
  NSURL *url = [NSURL URLFromString:tmpURL];
  </code></pre>
  
  <p>In this condition, it will make the app crash. I just want to make the <code>url</code> variable nil, not raise an error.</p>