title: Access NSMutableArray to an index with no value
body: <p>if i try to access a nsmutableArray with objectAtIndex:x  and if i have no object at this index, my app always crash.</p>
  
  <p>So my question is: how can i check, if there is something at this index, without crashing the app?</p>
  
  <p>I hope your understand my question.
  Thanks, Alex</p>