Swing/AWT/SWT :: BoxLayout Cannot Be Shared

I am trying to use a BoxLayout to make the equivalent of the vertical flow layout within a JPanel. I followed this tutorial: URL..., but was a little confused as to what the first argument in the constructor for the BoxLayout is (the tutorials that it was "the container that it manages," but I don't know what that means). All the examples I can find online just have the first argument as what ever kind of component they are trying to initialize using the BoxLayout, so I tried that but it produces the error "BoxLayout cannot be shared. "Here is my code:

//Main class
public class TestProject
{
public static void main(String[] args)
{
GUI mainWindow = new GUI();
mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainWindow.setSize(1000,1000);
mainWindow.setVisible(true);

[code]....