title: NSMutableArray removeAllObjects crash
body: <pre><code>Car class
  --------------
  price
  color
  </code></pre>
  
  <p>crash code is:</p>
  
  <pre><code>NSMutableArray *list = [[NSMutableArray alloc] init];
  Car *car = [[Car alloc] init];
  car.price = 10;
  car.color = 1;
  [list addObject:car];
  
  // some code
  
  [list removeAllObjects]; // Crash here
  </code></pre>
  
  <p>why crash, how can i resolve it.</p>
  
  <p>app exit with nothing output</p>