Tetris Game - Painting A Custom Shape

I am currently trying to make a Tetris game, so far I've got a class drawing that extends a JPanel as my canvas, a class Square that is 1 square(20 by 20) and a class LineShape that should draw 4 squares above each other. The class Frame way below is merely to set things up.

Currently i'm able to draw objects of my class Square, so I made a class LineShape that combines both and makes a 4Square long line.

My problem is when I try to draw an object of the class LineShape, it shows nothing, removes what is already painted(the square) and gives an error message:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at LineShape.<init>(Screen.java:96)
at Drawing.paint(Screen.java:79)

I put my classes together so you can copy paste and run, make a new screen to start, to see the problem occur, go to the drawing class's paint method and uncomment these lines:

// LineShape shape = new LineShape(this);
// shape.paint(g);

Below are my classes:

import javax.swing.*;
import java.awt.*;
public class Screen {
private Frame frame;
private Frame inputFrame;
private JPanel panel;

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