title: Angular mat tree with drag and drop
body: <p>I am using <strong><code>angular material flat tree</code></strong> for <strong><code>drag and drop</code></strong>. </p>
  
  <p>I implemented <strong><code>drag and drop</code></strong> using an example from stackblitz <a href="https://stackblitz.com/edit/mat-tree-with-drag-and-drop?file=app%2Ftree-flat-overview-example.ts" rel="nofollow noreferrer"><strong>link</strong></a>.</p>
  
  <p>I'm getting an error when i drag and drop parent node to child node.</p>
  
  <blockquote>
    <p>This is the error <strong>Cannot read property 'findIndex' of undefined</strong></p>
  </blockquote>
  
  <pre><code>Instead of showing that error i want to disable the action of DND of parent to child.
  </code></pre>
  
  <p>This is my <strong><a href="https://stackblitz.com/edit/mat-flat-tree-dnd?embed=1&amp;file=app/tree-flat-overview-example.ts&amp;view=preview" rel="nofollow noreferrer">work</a></strong> so far. 
  Thanks in advance.</p>