Null exception while send mail in Spring Boot?

My function to send email

    public void sendMailConfirm(String receiverAddress, int orderId) {
        Mail mail = new Mail();
        mail.setMailFrom(senderAddress);
        mail.setMailTo(receiverAddress);
        mail.setMailSubject(mailSubject);
        mail.setMailContent(mailContent + "\n\nOrder ID: " + orderId);

        mailService.sendMail(mail);
    }

My Mail class

public class Mail {
    private String mailFrom;

    private String mailTo;

    private String mailSubject;

    private String mailContent;

    private String contentType;

    private List<Object> attachments;

    getter & setter

I wrote this in an API. But when I test this. It returns a null exception. Please help me!

                //  send mail to notif success order
                Manager manager = new Manager();
                manager.sendMailConfirm(orderVM.getEmail(), order.getId());