NullPointerException?

[Xillez]

Hello, I have no clue what to do with this error below.

Error:
Exception in thread "main" java.lang.NullP ointerException
at main.Universe.< init>(Universe. java:31)
at main.Window.mai n(Window.java:9 1)

Window.java:

package main;

import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JMenuBar;
import javax.swing.JSlider;
import javax.swing.JToggleButton;

public class Window extends JFrame
{
	private static final long serialVersionUID = 1L;
	
	public static int wWidth = 1080;
	public static int wHeight = 720;
	public static boolean wVisible = false;
	
	//Input
	public static int nrMatter = 0;
	public static int nrAMatter = 0;
	
	public Window()
	{
		setTitle("Early universe sim");
		setSize(wWidth, wHeight);
		setVisible(true);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		getContentPane().setLayout(null);
		
		JMenuBar menuBar = new JMenuBar();
		setJMenuBar(menuBar);
		
		JToggleButton tglbtnRun = new JToggleButton("Run");
		menuBar.add(tglbtnRun);
		
		JSlider slider = new JSlider();
		slider.setMaximum(20);
		slider.setMinimum(10);
		slider.setExtent(1);
		menuBar.add(slider);
	}
	
	public void paint(Graphics g)
	{
		wVisible = true;
		while (wVisible)
		{
			//Check for wall collision before you move any objects
			for (int i = 0; i < Universe.id.length; i++)
			{
				if (Universe.xPos[i] == 0) { Universe.xSpeed[i] = Universe.xSpeed[i] * -1; }
				if (Universe.xPos[i] + Universe.cWidth == wWidth) { Universe.xSpeed[i] = Universe.xSpeed[i] * -1; }
				if (Universe.yPos[i] == 0) { Universe.ySpeed[i] = Universe.ySpeed[i] * -1; }
				if (Universe.yPos[i] + Universe.cHeight == wHeight) { Universe.ySpeed[i] = Universe.ySpeed[i] * -1; }
			}
			
			//Move
			for (int i = 0; i < Universe.id.length; i++)
			{
				Universe.xPos[i] = Universe.xPos[i] + Universe.xSpeed[i];
				Universe.yPos[i] = Universe.yPos[i] + Universe.ySpeed[i];
			}
			
			//Check for Collision here!
			
			//Painting the given amount of Matter at the current position
			for (int i = 0; i < Universe.nrMLoc; i++)
			{
				g.setColor(Color.RED);
				g.fillOval(Universe.xPos[i], Universe.yPos[i], Universe.cWidth, Universe.cHeight);
			}
			
			//Painting the given amount of Anti-Matter at the current position
			for(int i = Universe.nrAMLoc; i < Universe.nrAMLoc+Universe.nrMLoc; i++)
			{
				g.setColor(Color.BLUE);
				g.fillOval(Universe.xPos[i], Universe.yPos[i], Universe.cWidth, Universe.cHeight);
			}
			
			wVisible = false;
		}
	}
	
	public static void main(String[] args)
	{
		Universe.nrMLoc = 5;
		Universe.nrAMLoc = 5;
				
		//Initializing the early universes content
		Universe u = new Universe();
		
		//Construct window
		Window w = new Window();
	}
}

Universe.java:

package main;

import java.util.Random;

public class Universe 
{
	//Input
	public static int nrMLoc = 1;
	public static int nrAMLoc = 1;
	//Universe properties
	public static int cWidth = 25;
	public static int cHeight = 25;
	public static int maxX = Window.wWidth;
	public static int minX = 0;
	public static int maxY = Window.wHeight;
	public static int minY = 0;
	//Physics
	public static int gravity = 10;
	//Matter and Anti-Matter properties
	public static int[] id;
	public static int[] xPos;
	public static int[] yPos;
	public static int[] xSpeed;
	public static int[] ySpeed;
	
	public Universe()
	{
		Random r = new Random();
		for (int i = 0; i < nrMLoc+nrAMLoc; i++)
		{
			id[i] = i;
			xPos[i] = r.nextInt(Window.wWidth-2*cWidth)+cWidth;
			yPos[i] = r.nextInt(Window.wHeight-2*cHeight)+cHeight;
			xSpeed[i] = r.nextInt(30);
			ySpeed[i] = r.nextInt(30);
		}
	}
}

[Xillez]

Found it out. java is trying to convert from the data type null to int, and that is not possible. Just add integers in the array when you declare it.