2 Часа искал ошибку, нажал на проверку - всё правильно , задача решена, а при запуске программы ошибку выдает.. 
package com.javarush.task.task06.task0621;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/*
Родственные связи кошек
*/

public class Solution {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        String grandfatherName = reader.readLine();
        Cat catGrandfather = new Cat(grandfatherName);

        String grandmotherName = reader.readLine();
        Cat catGrandmother = new Cat(grandmotherName);

        String fatherName = reader.readLine();
        Cat catFather = new Cat(fatherName, null, catGrandfather);

        String motherName = reader.readLine();
        Cat catMother = new Cat(motherName, catGrandmother, null);

        String sonName = reader.readLine();
        Cat catSon = new Cat(sonName, catMother, catFather);

        String daughterName = reader.readLine();
        Cat catDaughter = new Cat(daughterName, catMother, catFather);

     ....
    }