NullPointerException error

package com.codegym.task.task08.task0824; /* Make a family */ import java.util.ArrayList; public class Solution { public static void main(String[] args) { //write your code here Human child1 = new Human("child1", false, 20); Human child2 = new Human("child2", true, 25); Human child3 = new Human("child3", true, 28); Human father = new Human("F1",true, 50); father.children.add(child1); father.children.add(child2); father.children.add(child3); Human mother = new Human("M1",false,40); mother.children.add(child1); mother.children.add(child2); mother.children.add(child3); Human grandF1 = new Human("Gf1", true,80); grandF1.children.add(father); Human grandF2 = new Human("Gf2", true, 70); grandF2.children.add(mother); Human grandM1 = new Human("GM1", false, 69); grandM1.children.add(father); Human grandM2 = new Human("GM2", false,80); grandM2.children.add(mother); System.out.println(grandF1); System.out.println(grandF2); System.out.println(grandM1); System.out.println(grandM2); System.out.println(father); System.out.println(mother); System.out.println(child1); System.out.println(child3); System.out.println(child3); } public static class Human { //write your code here String name; boolean sex; int age; ArrayList<Human> children; public Human (String name, boolean sex, int age){ this.name=name; this.sex=sex; this.age=age; } public Human (String name, boolean sex, int age, ArrayList<Human> children){ this.name=name; this.sex=sex; this.age=age; this.children=children; } public String toString() { String text = ""; text += "Name: " + this.name; text += ", sex: " + (this.sex ? "male" : "female"); text += ", age: " + this.age; int childCount = this.children.size(); if (childCount > 0) { text += ", children: " + this.children.get(0).name; for (int i = 1; i < childCount; i++) { Human child = this.children.get(i); text += ", " + child.name; } } return text; } } }