title: How to read 'unrecognized selector' error logs in obj-c / iphone apps
body: <p>i've got the following crash log error in my app:</p>
  
  <pre><code>-[NSNull length]: unrecognized selector sent to instance 0x194adc8
  </code></pre>
  
  <p>How do i read this? Does it mean that:</p>
  
  <ul>
  <li>I sent a 'length' message to a '<code>NSNull</code>' object?</li>
  <li>The 'length' method in the '<code>NSNull</code>' class crashed when trying to call a selector on another class?</li>
  </ul>
  
  <p>Also, if it is the former option, how can i get the stack trace to see which function caused this crash? That top line is the only error in my log.</p>
  
  <p>Thanks</p>