title: AG grid get selected rows at group index
body: <p><strong>Note:</strong> Grid data is grouped.</p>
  
  <p>I'm trying to get selected rows at specific group index.</p>
  
  <p>I tried <code>getDisplayedRowAtIndex(index).allLeafChildren</code> and loop through each node for selected true, this will give what I need, but only on first render.</p>
  
  <p>Any idea! How to achieve the same?
  Thanks</p>