Null Pointer Exception When adding a Class Object List to Another Class Object

Question

I am trying to add grades to a student object, this student object then has a list of these grades however I get an error

Exception in thread "main" java.lang.NullPointerException 
at Student.addGrade(Student.java:24)
at Test.main(Test.java:28))

I cannot find where my logic defeats me. This is my train of thought.

The Student class has a List gradeList and a method addGrade(Grade gr) which adds gr to the gradeList, Grade is a separate class object.

In the main I create a student object, add a new grade to the student, this grade is added to a list (addGrade Method) which is unique to that student.

This clearly is not the case and I simply cannot find why, can someone please point out my folly?

Test Class

import java.time.LocalDate;
import java.io.FileOutputStream;
import java.io.ObjectOutputStream;

public class Test {

    private static final String filepath="C:\\Users\\brads\\Desktop\\Serialized_Assignment3_File";

    public static void main(String[]args){

        Test test1 = new Test();

        Student s1 = new Student(121209, "John Smith", "Newsdale park", "Comp Sci", LocalDate.of(2000, 04, 10));
        test1.WriteStudentObjectToFile(s1);

        Student s2 = new Student(158009, "Sarah Byrne", "Shallow road", "Physics", LocalDate.of(2000, 01, 19));
        test1.WriteStudentObjectToFile(s2);

        s1.addGrade(new Grade("ICT343", LocalDate.of(2020, 6, 2), (short) 85));
        s1.addGrade(new Grade("ECC421", LocalDate.of(2020, 6, 6), (short) 73));
        s1.addGrade(new Grade("EGG101", LocalDate.of(2020, 6, 10), (short) 59));
        s1.addGrade(new Grade("EG101", LocalDate.of(2020, 6, 8), (short) 88));
        s1.addGrade(new Grade("EG101", LocalDate.of(2020, 6, 14), (short) 60));

        s2.addGrade(new Grade("ICT343", LocalDate.of(2020, 6, 2), (short) 58));
        s2.addGrade(new Grade("ECC421", LocalDate.of(2020, 6, 6), (short) 91));
        s2.addGrade(new Grade("EGG101", LocalDate.of(2020, 6, 10), (short) 59));
        s2.addGrade(new Grade("EG101", LocalDate.of(2020, 6, 8), (short) 74));
        s2.addGrade(new Grade("EG101", LocalDate.of(2020, 6, 14), (short) 80));

    }

    public void WriteStudentObjectToFile(Object serObj){
        try{
            FileOutputStream fileOut = new FileOutputStream(filepath);
            ObjectOutputStream objectOut = new ObjectOutputStream(fileOut);
            objectOut.writeObject(serObj);
            objectOut.close();
            System.out.println("The Object  was succesfully written to a file");
        }
        catch(Exception ex){
            ex.printStackTrace();
        }
    }
}

Student Class

import java.io.Serializable;
import java.time.LocalDate;
import java.util.List;

public class Student implements Serializable {

    private int id;
    private static final long serialVersionUID = 1L;
    private String name;
    private String address;
    private String course;
    private LocalDate dateOfBirth;
    private List<Grade> gradeList;

    public Student(int id, String name, String address, String course, LocalDate dateOfBirth){
        this.id = id;
        this.name = name;
        this.address = address;
        this.course = course;
        this.dateOfBirth = dateOfBirth;
    }

    public void addGrade(Grade gr){
        gradeList.add(gr);
    }

    public List<Grade> getGrades(){
        return gradeList;
    }

    @Override
    public String toString() {
        return new StringBuffer("Name: ").append(this.name).append(" Address : ").append(this.address).append(" Course : ").append(this.course).toString();
    }
}

Grade Class

import java.time.LocalDate;

public class Grade {

    private String moduleCode;
    private LocalDate date;
    private Short percentage;

    public Grade(String moduleCode, LocalDate date, Short percentage){
        this.moduleCode = moduleCode;
        this.date = date;
        this.percentage = percentage;
    }
}