title: Remove all children from a group without knowing the containing nodes
body: <p>check this out</p>
  
  <pre><code> Group g = new Group();
   GridPane grid = new GridPane(); // 
   g.getChildren().addAll(grid);
  </code></pre>
  
  <p>now my question is how do i remove this "grid" from "g" without specifying "grid"
  like something like this</p>
  
  <pre><code>g.getChildren().removeAll(null); //i do not know what to insert here?
  </code></pre>
  
  <p>thanks in advance</p>