Ошибка TreeSet, Set в Java [дубликат]

У меня возникает ошибка:

Exception in thread "main" java.lang.NullPointerException
    at Department.getWorkerWithMaxSalary(Department.java:19)
    at Table.main(Table.java:42)

вот мой код Department:

import java.util.*;
public class Department implements Comparable<Department>{
    private String nameOfTheDepartment;
    private Worker directorOfTheDepartment;
    private Set<Worker> workerList;
    private Worker chairman;
    private Set<Department> listOfDepartments;

    public Department(String nameOfTheDepartment, Worker directorOfTheDepartment, Set<Worker> setOfDepartament) {
        this.nameOfTheDepartment = nameOfTheDepartment;
        this.directorOfTheDepartment = directorOfTheDepartment;
        this.workerList = workerList;
    }
    public Worker getWorkerWithMaxSalary(){
        Worker workerWithMaxSalary = directorOfTheDepartment;
       **Department.java:19** for (Worker worker : workerList){
            if(workerWithMaxSalary.getSalary()<worker.getSalary())
                workerWithMaxSalary = worker;
        }
        return  workerWithMaxSalary;
    }
    public Set<Worker> getFullWorkerList(){
        Set<Worker> workerList = new TreeSet<Worker>(new SalaryComparator());
        workerList.add(this.chairman);
        Iterator iter = listOfDepartments.iterator();
        while (iter.hasNext()){
            Department department = (Department)iter.next();
            workerList.add(department.getDirectorOfTheDepartment());
            workerList.addAll(department.getWorkerList());
        }
        return workerList;
    }
    @Override
    public int compareTo(Department b) {
        int result = this.nameOfTheDepartment.compareTo(b.nameOfTheDepartment);
        if (result != 0) {
            return result;
        }
        return this.nameOfTheDepartment.compareTo(b.nameOfTheDepartment);
    }
    public void addWorker(Worker worker){
        workerList.add(worker);
    }

    public String getNameOfTheDepartment() {
        return nameOfTheDepartment;
    }
    public void setNameOfTheDepartment(String nameOfTheDepartment) {
        this.nameOfTheDepartment = nameOfTheDepartment;
    }
    public Worker getDirectorOfTheDepartment() {
        return directorOfTheDepartment;
    }
    public void setDirectorOfTheDepartment(Worker directorOfTheDepartment) {
        this.directorOfTheDepartment = directorOfTheDepartment;
    }
    public Set<Worker> getWorkerList() {
        return workerList;
    }
    public void setWorkerList(Set<Worker> workerList) {
        this.workerList = workerList;
    }

}

Вот код Firm:

import java.util.*;
public class Firm  {
    private String nameOfFirm;
    private Worker chairman;
    private Set<Department> listOfDepartments;
    public Firm(String nameOfFirm, Worker chairman) {
        this.nameOfFirm = nameOfFirm;
        this.chairman = chairman;
        this.listOfDepartments = listOfDepartments;
    }

    public Set<Department> findAllDepartmentsWihLowDirectorSalary(){
        Set<Department> departmentList = listOfDepartments;
        Iterator<Department> iter = listOfDepartments.iterator();
        while (iter.hasNext()){
            Department department = iter.next();
            if(department.getDirectorOfTheDepartment()!=department.getWorkerWithMaxSalary())
                departmentList.add(department);
        }
        return departmentList;
    }

    public Worker getWorkerWithMaxSalary(){
        Worker workerOfTheFirmWithMaxSalary = chairman;
        for (Department department : listOfDepartments){
            if(workerOfTheFirmWithMaxSalary.getSalary() < department.getWorkerWithMaxSalary().getSalary())
                workerOfTheFirmWithMaxSalary = department.getWorkerWithMaxSalary();
        }
        return  workerOfTheFirmWithMaxSalary;
    }

    public void addDepartment(Department department) {
        listOfDepartments.add(department);
    }
    public String getNameOfFirm() {
        return nameOfFirm;
    }
    public void setNameOfFirm(String nameOfFirm) {
        this.nameOfFirm = nameOfFirm;
    }
    public Worker getChairman() {
        return chairman;
    }
    public void setChairman(Worker chairman) {
        this.chairman = chairman;
    }
    public Set<Department> getListOfDepartments() {
        return listOfDepartments;
    }
    public void setListOfDepartments(Set<Department> listOfDepartments) {
        this.listOfDepartments = listOfDepartments;
    }
}

Вот Компаратор:

import java.util.Comparator;
public class SalaryComparator implements Comparator<Worker> {
    SalaryComparator() { }
        public int compare(Worker w1, Worker w2) {
            int result = w1.getWorkerName().compareTo(w2.getWorkerName());
            return result != 0 ? result : (int)(w1.getSalary() - w2.getSalary());
        }
    }

Вот код Worker:

public class Worker {
    private String workerName;
    private String workerSurname;
    private int salary;
    public Worker(String workerName, String workerSurname, int salary) {
        this.workerName = workerName;
        this.workerSurname = workerSurname;
        this.salary = salary;
    }
    @Override
    public String toString() {
        return "[Name=" + workerName + ", Surname=" + workerSurname + ", salary=" + salary + "]";
    }

    public String getWorkerName() {
        return workerName;
    }
    public void setWorkerName(String workerName) {
        this.workerName = workerName;
    }
    public String getWorkerSurname() {
        return workerSurname;
    }
    public void setWorkerSurname(String workerSurname) {
        this.workerSurname = workerSurname;
    }
    public int getSalary() {
        return salary;
    }
    public void setSalary(int salary) {
        this.salary = salary;
    }
}

Вот класс, где создаются списки и прочее:

import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
public class Table {
    public static void main(String[] args){
        Worker chairman = new Worker("Jordj", "Valit", 3000);

        //DevDepartment
        Worker devDepartmentDirector = new Worker("Joe", "Hanh", 13000);
        Worker worker1 = new Worker("Mike", "Shinoda", 2500);
        Worker worker2 = new Worker("Chester", "Bennington", 1500);
        Worker worker3 = new Worker("Brad", "Delson", 1200);
        //EngineringDepartment
        Worker engineringDepartmentDirector = new Worker("Sharon", "del Adel", 600);
        Worker worker4 = new Worker("Robert", "Westerholt", 600);
        Worker worker5 = new Worker("Jeroen", "van Veen", 2000);
        Worker worker6 = new Worker("Ruud", "Jolie", 1300);
        Set<Worker> setf1 = new TreeSet<Worker>(new SalaryComparator());
        setf1.add(worker1);
        setf1.add(worker2);
        setf1.add(worker3);
        Set<Worker> setf2 = new TreeSet<Worker>(new SalaryComparator());
        setf2.add(worker4);
        setf2.add(worker5);
        setf2.add(worker6);

        Department f1 = new Department("DevDepartment", devDepartmentDirector, setf1);
        Department f2 = new Department("EngineringDepartment", engineringDepartmentDirector, setf2);
        Set<Department> setofDepartment = new TreeSet<Department>();
        setofDepartment.add(f1);
        setofDepartment.add(f2);
       **Table.java:42** Worker workerWithMaxSalary = f1.getWorkerWithMaxSalary();
        System.out.println("Максимальну зарплату має: "+workerWithMaxSalary.getWorkerName()+" "+workerWithMaxSalary.getWorkerSurname()+ " - "+ workerWithMaxSalary.getSalary() + " грн");
        System.out.println(" ");

        for(Department department: setofDepartment){
            System.out.println("Директор DevDepartment : " + devDepartmentDirector);
            System.out.println("Директор EngineringDepartment" + engineringDepartmentDirector);
            System.out.println("Начальник : " + chairman);
            System.out.println(" ");
            System.out.println("Список усіх працівників компанії :");
            for(Worker worker: department.getFullWorkerList()){
                System.out.println(worker.getWorkerName()+" "+worker.getWorkerSurname());
            }
        }
    }
}

Подскажите, пожалуйста, как решить эту проблему? Строки, где возникает ошибка обозначены. Буду очень благодарна.