Why does an initialized array of doubles mark a nullPointer?

The following code does not flag compilation errors, but when I call the method darDeAlta()and enter data, it flags the following error:

Exception in thread "main" java.lang.NullPointerExceptionIn the line d[i] = y.nextDouble();(within the method) and in the line bandera = e.darDeAlta();(within the main), I already initialized the array of doubles with nulland I don't understand why it gives me that error.

public class Ejercicio {

Map<Integer, Alumno> alumnos = new TreeMap<>();

public boolean darDeAlta() {
    Scanner y = new Scanner(System.in);
    boolean b = false;
    double d[] = null;

    System.out.println("Introduzca el nombre del estudiante");
    String n = y.nextLine();
    System.out.println("Introduzca su matricula (de 5 digitos)");
    int m = y.nextInt();
    System.out.println("Introduzca las calificaciones: \n");
    for (int i = 0; i < 5; i++) {
        System.out.println("Introduzca la calificacion");
        d[i] = y.nextDouble();//marca error aqui

    }

    if (alumnos.containsKey(m)) {
        System.out.println("Esa matricula ya existe, ingrese otra matricula");

    } else {
        alumnos.put(m, new Alumno(n, m, d));
        System.out.println("Se han dado de alta los datos :)");
        System.out.println("Desea ingresar mas alumnos?");
        String s = y.nextLine();
        if (s.equals("si")) {
            b = true;
        } else {
            b = false;
        }
    }
    return b;

}
    public static void main(String[] args) {

    Scanner x = new Scanner(System.in);
    boolean bandera = false;

    Ejercicio e = new Ejercicio();

    do {

        System.out.println("[a]ltas" + "\n" + "[b]ajas" + "\n" + "[c]onsultas" + "\n" + "[m]odificaciones" + "\n" + "[s]alir");
        System.out.println("\n Seleccione una opcion");
        String opcion = x.next();
        if (opcion.compareTo("a") == 0) {
            bandera = e.darDeAlta();//marca error aqui
        }
        if (opcion.compareTo("b") == 0) {
            e.darDeBaja();
            if (opcion.compareTo("c") == 0) {
                e.consultas();
                if (opcion.compareTo("m") == 0) {
                    e.modificaciones();
                    if (opcion.compareTo("s") == 0) {
                        bandera = false;

                    }
                }
            }
        }
    } while (bandera);
}

}