Updated Object ArrayList not saving to txt file

Question

I have a student class has a name,id and an arraylist of modules. I then have an arraylist of students that I write to a txt file. When I initially create a studentList using the constructor to pass the name and ID it saves just fine but when I update the student list and add some modules to the students it gave me a seriazable error

Student Class:

`public class Student implements Serializable{
//each student has an arraylist of modules
    
    private String name = null;     //students name
    private int ID = 0;             //students ID number
    private String DOB = null;      //Students Date of Birth
    
    private ArrayList <Module> modules;         //creating the students module arraylist
    
    public Student(String name,int ID, String DOB) {
        this.setName(name);
        this.setID(ID);
        this.setDOB(DOB);
        setModules(new ArrayList <Module> ());          //initialising the arraylist
    }
    `

StudentList Class:

    package application;

import java.util.ArrayList;
import java.util.List;

//student arraylist
public class StudentList {

    
    private ArrayList <Student> students;           //creating the arraylist
    
    
    
    //constructor
    public StudentList() {
        students = new ArrayList <Student> ();          //initialising the arraylist
    }
    
    
    public Student findStudent (Student student) {
        for(Student st: students) {
            if(st.equals(student)) {
                return st;
            }
            
        }
        return student;
    }
    
    //getter
    public ArrayList<Student> getList (){
        return students;
    }
    
    public void addStudent(Student stu)
    {
        students.add(stu);
    }
    
    public void removeStudent(Student stu) {
        students.remove(stu);
    }
    
    //return an element of the students array
    public Student getStudent(int i)
    {
        if ((i>-1) && (i < students.size()))            //making sure a valid index was entered
        return students.get (i);
        return null;
    }
    
    //getter for the size of the array
    public int getSize (){
        return students.size();
    }

//method to clear the arraylist
    public void clear() {
        students.clear();
        
    }

//method to add a list of type Student
    public void addAll(List<Student> students2) {
        
        for(Student student: students2) {
            students.add(student);
            System.out.println("Added " + student);
        }
    }
    
    public Student returnElement(int i) {
        Student element = students.get(i);
        return element;
    }

    
}

Controller:

    package application;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
import java.util.List;

public class StudentController {
    
    
    private StudentList stu;        //creating instance of the StudentList class
    
    //constructor 
    public StudentController() {
        stu = new StudentList();
    }

    //adding a student to this list
    public void addStudentToList(Student s) {
        stu.addStudent(s);
    }
    
    public int getSize() {                  //getting the size of the arraylist
        return  stu.getSize();
    }
    
    
    public void addStudentToList(String studentName,int studentID, String studentDOB)
    {
        
        Student student = new Student(studentName, studentID, studentDOB);
        stu.addStudent(student);       //adding the student to the student list
    }
    
        //add students module
        public void addModule(String name, int grade, Student student) {
            Module module = new Module(name,grade);
            stu.findStudent(student).addModule(module);         //adding the module to that Student
            stu.addStudent(stu.findStudent(student));           //now adding that student to our arrayliust]
            
        }
    
    public void saveMethod() {          //method to save to the text file
        try {
            File file  = new File("StudentData.txt");
            //File to store student data
            ObjectOutputStream os = new ObjectOutputStream(new FileOutputStream(file));
            os.writeObject(stu.getList());
            os.close();
        }

        catch(FileNotFoundException e1){
            e1.printStackTrace();
        }
        catch(IOException e1) {
            e1.printStackTrace();
        }
    }
        
    
    
    
    public void load() {                    //method to load the contents of the text file onto my arraylist
        

        try {
            FileInputStream fis = new FileInputStream("StudentData.txt");
            ObjectInputStream ois = new ObjectInputStream(fis);
            List<Student> students = (List<Student>) ois.readObject();
            stu.clear();
            stu.addAll(students);
            ois.close();

        } 
        catch (FileNotFoundException e1) {
            e1.printStackTrace();
        }
        catch (IOException e1) {
            e1.printStackTrace();
        }

        catch (ClassNotFoundException e1) {
            e1.printStackTrace();
        }

        
    }
            
            
    
        
    public Student returnElement(int i) {
        return stu.getStudent(i);
        
    }
        
    //getter for studentList
    public String getListStudent()
    {
        String allStudents="\0";
        for (int i = 0;i<stu.getSize();i++)         //looping through the array
        {
            allStudents = allStudents+stu.getStudent(i);    
        }   
        return allStudents;
    }

    
    public void removeStudent(int ID) {
        Student studentToRemove = null;
        for(Student student:stu.getList()){                 //iterate over the student list
            if(student.getID()==ID) {                       //compring ID entered to the ID in student arraylist
                studentToRemove=student;
                stu.removeStudent(studentToRemove);         //then remove that student
            }       
        }
        
    }
    
}

The save method is what gives this error: java.io.NotSerializableException: application.Module at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1193)