NullPointerException Adding JPanel To JPanel

I have a Main() class that extends JPanel and I'm trying to add Card() that also extends JPanel.When I add a Card() or Main() object to the JFrame directly, it works fine. The problem arises when I try to add a Card() object to the Main() object.

I edited the code a little bit to make it easier to read so hopefully it's fine.

Main() code:
public class Main extends JPanel implements Runnable {
// HARDCODED OPTIONS
public static final int WIN_WIDTH = 1200;
public static final int WIN_HEIGHT = 800;

public static final int GRID_X = 9;
public static final int GRID_Y = 5;

[Code] ...