Exception in thread "main" java.lang.NullPointerException at java.desktop/java.awt.Container.addImpl(Container.java:1117)

Давно не игрался Java-й , хотел сделать простенький калькулятор на JSwing-е, получаю такие ошибки при компиляции такого кода :

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import java.awt.GridLayout;
import java.awt.BorderLayout;




public class Calculator extends JFrame
{

    
                 



JPanel windowContent;
JTextField displayField;
JButton button0;
JButton button1;
JButton button2;
JButton button3;
JButton button4;
JButton button5;
JButton button6;
JButton button7;
JButton button8;
JButton button9;
JButton buttonPoint;

JButton buttonEqual;
JPanel pl;

Calculator()
{
    windowContent = new JPanel();
    
    BorderLayout bl = new BorderLayout();
    windowContent.setLayout(bl);
    
    displayField = new JTextField(30);
    windowContent.add("North", displayField);
    
    button0 = new JButton("0");
    button0 = new JButton("1");
    button0 = new JButton("2");
    button0 = new JButton("3");
    button0 = new JButton("4");
    button0 = new JButton("5");
    button0 = new JButton("6");
    button0 = new JButton("7");
    button0 = new JButton("8");
    button0 = new JButton("9");
    
    buttonPoint = new JButton(".");
    buttonEqual = new JButton("=");
    
    pl = new JPanel();
    
    GridLayout gl = new GridLayout(4, 3);
    pl.setLayout(gl);
    
    pl.add(button0);
    pl.add(button1);
    pl.add(button2);
    pl.add(button3);
    pl.add(button4);
    pl.add(button5);
    pl.add(button6);
    pl.add(button7);
    pl.add(button8);
    pl.add(button9);
    
    pl.add(buttonPoint);
    pl.add(buttonEqual);
    
    windowContent.add("Center", pl);
    
    JFrame frame = new JFrame("Calculator");
    frame.setContentPane(windowContent);
    
    frame.pack();
    frame.setVisible(true);
    
    }

    public static void main(String[] args)
    {
      Calculator calc = new Calculator();
    }
 }


Exception in thread "main" java.lang.NullPointerException
at java.desktop/java.awt.Container.addImpl(Container.java:1117)
at java.desktop/java.awt.Container.add(Container.java:436)
at Calculator.<init>(Calculator.java:66)
at Calculator.main(Calculator.java:91)

Версия Java : 8 Update 271 (build 1.8.0_271-b09)

Компилировал в Eclipse версии 2020-09 (4.17.0) Build id: 20200910-1200