Matrix multiplication in BlueJ (Java)

can anyone find me the mistake in the following code?

This is the multiplication of matrices that contain fractions. It shows me a NullPointerException and underscores the line with z[i][j]. The scitaj and vynasob methods are already tested and the error should not be there. I am attaching a picture. If you need to see a larger part of the code for understanding, tell me.

Thank you very much :)

matica - matrix zlomok - fraction nasobenie, vynasob - multiplication PocetRiadkov - number of rows PocetStlpcov - number of columns scitaj - +

public Matica nasobenie() {
    Matica xs = new Matica(new Zlomok[][]
    {
        {new Zlomok(1, 1), new Zlomok(2, 1), new Zlomok(3, 1)},
        {new Zlomok(4, 1), new Zlomok(5, 1), new Zlomok(6, 1)}
    });
        
    Matica ys = new Matica(new Zlomok[][]
        {
            {new Zlomok(7, 1), new Zlomok(8, 1)},
            {new Zlomok(9, 1), new Zlomok(10, 1)},
            {new Zlomok(11, 1), new Zlomok(12, 1)}
        });
    
    Zlomok [][] z = new Zlomok[xs.getPocetRiadkov()][ys.getPocetStlpcov()];
    System.out.println(z);
    for (int i = 0; i < z.length; i++) {
        for (int j = 0; j < z[0].length; j++) {
            for (int k = 0; k < xs.getPocetStlpcov(); k++) {
                z[i][j] = z[i][j].scitaj(xs.getPrvok(i, k).vynasob(ys.getPrvok(k, j)));
            }
            System.out.println();
        }
    }
    Matica vysledok = new Matica(z);
    return vysledok;        
}