The method returns null pointer exception the null pinter is from serialized objects, I couldn't figure out the error yet

Question

This method is in a class without implementing serializable interface

   public String FindVetsMedicalAppointments(String id){   MedicalBooking.getInstance().deserializeMedicalBooking();
    for (int i = 0; i < MedicalBooking.mediBookings.size(); i++) {
        MedicalBooking.getInstance().deserializeMedicalBooking();
        if(MedicalBooking.mediBookings.get(i).getVetId().equals(id)){
            System.out.println(MedicalBooking.getInstance().mediBookings.get(i).toString());
        }
    }
    return "END OF APPOINTMENTS";
 }

This method is in a class implementing serializable interface

  public void deserializeMedicalBooking(){
    try{
            FileInputStream fileIn = new FileInputStream("Medical Appointments.txt");
    ObjectInputStream in = new ObjectInputStream(fileIn);
    mediBookings = (ArrayList<MedicalBooking>) in.readObject();
    } catch (ClassNotFoundException | IOException i) {}
}