JPanel added but not displayed “in time”

Question

I have a JFrame that displays JPanels depending on the MenuItem you click. It works, but now I need to call a method once a JPanel is added to the frame and it is being show

Answer 1

It seems it is solved thanks to invokeLater:

public void methodCalledOnceDisplayed() {
        SwingUtilities.invokeLater( new Runnable() {
            @Override
            public void run() {
                chartPanel.repaint();
                chartPanel.getChartRenderingInfo().getPlotInfo().getSubplotInfo(0).getDataArea();
                chartPanel.getChartRenderingInfo().getPlotInfo().getSubplotInfo(1).getDataArea();
            }
        });
    }

Now there is no IndexOutOfBoundsException: Index: 0, Size: 0

An explanation of why is this happening would be great