почему вываливается NPE?

package com.javarush.task.task08.task0824; import java.util.ArrayList; import java.util.List; /* Собираем семейство */ public class Solution { public static void main(String[] args) { ArrayList<Human> children = new ArrayList<>(); Human gpa1 = new Human(); Human gpa2 = new Human(); Human gma1 = new Human(); Human gma2 = new Human(); Human pa = new Human(); Human ma = new Human(); Human ch1 = new Human(); Human ch2 = new Human(); Human ch3 = new Human(); gpa1.name = "ded1"; gpa1.age = 72; gpa1.sex = true; gpa1.children.add(pa); gma1.name = "ba1"; gma1.age = 58; gma1.sex = false; gma1.children.add(pa); gpa2.name = "ded2"; gpa2.age = 68; gpa2.sex = true; gpa2.children.add(ma); gma2.name = "ba2"; gma2.age = 58; gma2.sex = false; gma2.children.add(ma); pa.name = "pa"; pa.age = 40; pa.sex = true; pa.children.add(ch1); pa.children.add(ch2); pa.children.add(ch3); ma.name = "ma"; ma.age = 40; ma.sex = false; ma.children.add(ch1); ma.children.add(ch2); ma.children.add(ch3); } public static class Human { String name; boolean sex; int age; ArrayList<Human> children; 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; } } }