title: replace object in mutable array
body: <p>This should be rather easy and probably I'm just missing a small thing:
  I have an array of images called defaultImages</p>
  
  <pre><code>NSMutableArray *defaultLetters;
  </code></pre>
  
  <p>then I add the letters needed (array length then is 42)</p>
  
  <p>later I'm trying to replace one of the images in the array with another image. Like this:</p>
  
  <pre><code>[defaultLetters replaceObjectAtIndex:0 withObject:croppedPhoto];
  </code></pre>
  
  <p>but I'm getting an error saying: "-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance 0x180ae190"
  is there a step I'm missing?</p>