java.lang.IllegalArgumentException: input == null! finding sprite sheet

Question

I was trying to locate a sprite-sheet using this code. I tried for like 3 days but i can't really find the problem. ¿Can anyone tell me where is the error?

public class HojaSprites {
    public final int[] pixeles;
    private final int alto;
    private final int ancho;
    
    //colección de hojas de sprites  
    
    public static HojaSprites SPACE = new HojaSprites("/assets/spaceships.png", 192, 192); //192 x 192

    //fin de colección
    
    
    public HojaSprites(final String ruta, final int ancho, final int alto){
    this.ancho=ancho;
    this.alto=alto;
    
    pixeles= new int[ancho*alto];
    
    BufferedImage imagen; 
    try{
        imagen = ImageIO.read(HojaSprites.class.getResource(ruta));
        imagen.getRGB(0,0,ancho,alto,pixeles,0,ancho);}
    catch (IOException e) {}
    
    }
    
    
    public int getAncho(){
    return ancho;
    }
    
    public int getAlto(){
    return alto;
    }
}

I still getting this errors. May it even be an another class problem?

error