title: How to force NSOutlineView to call outlineView:numberOfChildrenOfItem
body: <p>After my method finishes, NSOutlineView calls outlineview:child:ofIndex: for reasons that are not clear to me.</p>
  <p>It does not call outlineview:numberOfChildrenOfItem first.</p>
  <p>Since I deleted an item from my data in the method, I get an error in outlineview:child:ofIndex: when the index exceeds the number of remaining children.</p>
  <p>Any work-around would be an acceptable answer.</p>