java.lang.NullPointerException: Cannot invoke "java.awt.image.BufferedImage.getSubimage(int, int, int, int)" because "this.tileSet" is null

Question

I am trying to code a little RPG and I'm stuck at getting images to my game. I have a problem with loading a tileset. I wrote a tileSet class, but I the tileset is always null.

Check my code, maybe someone could find the problem.

TileSet class:

import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.*;
import javax.imageio.ImageIO;

public class TileSet {

public static final int TILEWIDTH = 64, TILEHEIGHT = 64;

  private BufferedImage[] tiles;
  
  BufferedImage tileSet;
  
  public TileSet(String path, int sizeX, int sizeY) {
    tiles = new BufferedImage[sizeX * sizeY];  
   try {
       tileSet = ImageIO.read(TileSet.class.getResource(path));
    } catch (IOException e) {
      e.printStackTrace();
      return;
    }

    int i = 0;
    for(int y = 0; y < sizeY; y++) {
      for(int x = 0; x < sizeX; x++) {
        tiles[i++] = tileSet.getSubimage(x * (TILEWIDTH + 3), y * (TILEHEIGHT + 3),
              TILEWIDTH, TILEHEIGHT);
      }
    }
  }
  public void renderTile(Graphics g, int tileNum, int x, int y){
    g.drawImage(tiles[tileNum], x, y, TILEWIDTH, TILEHEIGHT, null);
  }
}

Main class:

package rpgame;

import java.awt.Canvas;
import java.awt.Graphics;
import java.awt.image.BufferStrategy;
import java.awt.image.BufferedImage;

public class RPGame implements Runnable {
 public static final int FPS = 60;
 public static final long maxLoopTime = 1000 / FPS;
 public static final int SCREEN_WIDTH = 640;
 public static final int SCREEN_HEIGHT = 640;
  
 public Screen screen; 

  public static void main(String[] arg) {
  RPGame game = new RPGame();
  new Thread(game).start();
 }
 Level level;
 @Override
 public void run() {
  long timestamp;
  long oldTimestamp;
  BufferedImage playerImages;
  screen = new Screen("Game", SCREEN_WIDTH, SCREEN_HEIGHT);

    TileSet tileSet = new TileSet("/tiles/rpg.webp", 12, 12);
    level = new Level("/level/level1.txt", tileSet);


  while(true) {
   oldTimestamp = System.currentTimeMillis();
   update();
   timestamp = System.currentTimeMillis();
   if(timestamp-oldTimestamp > maxLoopTime) {
    System.out.println("Wir sind zu spät!");
    continue;
   }
   render();
   timestamp = System.currentTimeMillis();
   System.out.println(maxLoopTime + " : " + (timestamp-oldTimestamp));
   if(timestamp-oldTimestamp <= maxLoopTime) {
    try {
     Thread.sleep(maxLoopTime - (timestamp-oldTimestamp) );
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
  }
 }

 void update() {
  try {
    Thread.sleep(15);
  } catch (InterruptedException e) {
    e.printStackTrace();
  };
 }
 BufferStrategy bs;
 Graphics g;
 void render() {
  Canvas c = screen.getCanvas();
  // c.setBackground(Color.blue);
  bs = c.getBufferStrategy();
  if(bs == null){
   screen.getCanvas().createBufferStrategy(3);
   return;
  }
  g = bs.getDrawGraphics();
  //Clear Screen
  g.clearRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
  level.renderMap(g);
  bs.show();
  g.dispose();
 }

}

I have tried to change the path of the image file / working with File class.