java.lang.NullPointerExpectation:null (in java.awt.Container) when adding a JPanel

Hi i am having an issue with this error "java.lang.NullPointerExpectation:null (in java.awt.Container)", it wasn't happening until i added the jpanel any solutions?

Code :

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package folderCreating;
import java.io.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
 
public class window extends JFrame{
 
    private JTextField textField1;
    private JButton button1;
    private JTextField textField2;
    private JButton button2;
    private JButton button3;
    private JButton button4;
    public static String fileInput;
    public static String folderInput;
 
    public window(){
 
        super("File and Folder Manager");  
 
        setLayout(new FlowLayout());
 
        JPanel p1 = new JPanel();
 
        p1.add(textField1);
        p1.add(button2);
        add(p1);
 
        textField2 = new JTextField(20);
        add(textField2);
 
        button1 = new JButton("Create Folder");
        add(button1);
 
        textField1 = new JTextField(20);
        add(textField1);
 
        button2 = new JButton("Create File");
        add(button2);
}

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which line produces the error?

Step through the code as the computer would. First call the constructor:
public window(){
so far so good. continue...
setLayout(new FlowLayout());
JPanel p1 = new JPanel();
ok.... go on.......
p1.add(textField1);
Stop. textField1 has not been initialized. (can not add 'null' to a JPanel)
textField1 must have been assigned a value first. This is not the only variable with the problem.

well according to your code the compiler is right, you have not initialized the variable textField1,
you need to create a new object with the new keyword, and then you can add it.
edit : the order of which is important, first make object, then add it.