title: javafx GridPane retrieve specific Cell content
body: <p>I want to retrieve the content of one specific cell in a Gridpane. I have put buttons in the cells with the </p>
  
  <pre><code>setConstraints(btt , 0 ,1 ) 
  
  setConstraints(btt , 0 ,2 )
  
  getChildren().add....
  </code></pre>
  
  <p>In my case the <code>GridPane.getChildren.get(10)</code> is not good. I want to go directly to the cell(4,2) and get its content.</p>