Controls added to "JRootPane" cannot be designed

[WolfgangHG]

I have old code that adds controls directly to the JRootPane.
WindowBuilder cannot handle this and just displays a blank form.

public MainFrame2()
{
  ...
  
  GridBagLayout layout = new GridBagLayout();
  this.getRootPane().setLayout(layout);
  
  JLabel lblNewLabel = new JLabel("New label");
  layout.setConstraints(lblNewLabel, new GridBagConstraints(0, 0, 1, 1, 0, 0, GridBagConstraints.WEST,
      GridBagConstraints.NONE, new Insets(10, 10, 0, 0), 0, 0));
  this.getRootPane().add(lblNewLabel);
  
  JButton btnNewButton = new JButton("New button");
  layout.setConstraints(btnNewButton, new GridBagConstraints(1, 0, 2, 1, 1.0, 0, GridBagConstraints.WEST,
      GridBagConstraints.HORIZONTAL, new Insets(10, 0, 0, 10), 0, 0));
  this.getRootPane().add(btnNewButton);

}

Is this something that should be supported and that might be worth a bug report? Or is this code a "don't" approach and I should better use a "ContentPane" panel?

It works if I do it like this:

public MainFrame3()
{
  ...
  
  contentPane = new JPanel();
  contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
  setContentPane(contentPane);
  
  GridBagLayout layout = new GridBagLayout();
  contentPane.setLayout(layout);
  
  JLabel lblNewLabel = new JLabel("New label");
  layout.setConstraints(lblNewLabel, new GridBagConstraints(0, 0, 1, 1, 0, 0, GridBagConstraints.WEST,
      GridBagConstraints.NONE, new Insets(10, 10, 0, 0), 0, 0));
  contentPane.add(lblNewLabel);
  
  JButton btnNewButton = new JButton("New button");
  layout.setConstraints(btnNewButton, new GridBagConstraints(1, 0, 1, 1, 1.0, 0, GridBagConstraints.WEST,
      GridBagConstraints.HORIZONTAL, new Insets(10, 0, 0, 10), 0, 0));
  contentPane.add(btnNewButton);
}

Attached is a sample project - "MainFrame2.java" is the broken approach, "MainFrame3.java" the working solution.
WindowBuilderSample.zip