Что за ошибка?

Подскажите плиз, что тут не так???? Выдает такую ошибку Exception in thread "main" java.lang.NullPointerException at com.javarush.task.task08.task0824.Solution$Human.toString(Solution.java:68) at java.base/java.lang.String.valueOf(String.java:2788) at java.base/java.io.PrintStream.println(PrintStream.java:897) at com.javarush.task.task08.task0824.Solution.main(Solution.java:26)

public class Solution {
    public static void main(String[] args) {

        Human child1 = new Human("asda", false, 15, null, null, null);
        Human child2 = new Human("Gnsadasom", true, 11, null, null, null);
        Human child3 = new Human("Meloch", true, 10, null, null, null);


        Human Mamka = new Human("Mamka", false, 35, child1, child2, child3);
        Human Papka = new Human("Papka", true, 40,  child1, child2, child3);


        Human ded1 = new Human("Ded1", true, 65, Mamka, null, null);
        Human babka1 = new Human("Babka1", false, 55, Mamka, null, null);
        Human ded2 = new Human("Ded2", true, 59, Papka, null, null);
        Human babka2 = new Human("Babka2", false, 57, Papka, null, null);

        System.out.println(ded1);
        System.out.println(ded2);
        System.out.println(babka1);
        System.out.println(babka2);
        System.out.println(Mamka);
        System.out.println(Papka);
        System.out.println(child1);
        System.out.println(child2);
        System.out.println(child3);


    }

    public static class Human {
     String name;
     boolean sex;
     int age;
     ArrayList<Human> children = new ArrayList<>();

     public Human(String name, boolean sex, int age, Human children, Human children1, Human children2){
         this.name = name;
         this.sex =sex;
         this.age = age;
         this.children.add(children);
         this.children.add(children1);
         this.children.add(children2);

      }


        public String toString() {
            String text = "";
            text += "Имя: " + this.name;
            text += ", пол: " + (this.sex ? "мужской" : "женский");
            text += ", возраст: " + this.age;

            int childCount = this.children.size();
            if (childCount > 0) {
                text += ", дети: " + this.children.get(0).name;

                for (int i = 1; i < childCount; i++) {
                    Human child = this.children.get(i);
                    text += ", " + child.name;
                }
            }
            return text;
        }
    }

}