title: Anyone know what cause this crash:"[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector"?
body: <p>Recently I use NSTextTable draw table. I want change some attribute of selected block,so save selected paragraphs into dictionary. After I copy the block of paragraph, once I call method [NSTextTableBlock setBorderColor:], raise an error:</p>
  
  <pre><code>-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance 0x6000002442c0
  </code></pre>
  
  <p>I try many times, finally found a phenomenon, which is if I don't copy block, setBorderColor: is OK.
  Anyone know relation of [NSTextTableBlock copy] and this error, why [NSTextTableBlock setBorderColor:] cause this error? I'm quite confuse about this error. Thanks in advance.</p>