PriorityQueue

What will happen if you run this code?

public class PriorityQueueDemo {
    public static void main(String[] args) {
        Queue<Person> queue = new PriorityQueue();
        queue.add(new Person(90);
        queue.add(new Person(85));
        queue.add(new Person(95);
        queue.add(null);

        System.out.println(queue.poll().getMathScore());
        System.out.println(queue.poll().getMathScore());
        System.out.println(queue.poll().getMathScore());
    }
}

class Person {
    private int mathScore;

    public Person(int mathScore) {
        this.mathScore = mathScore;
    }

    // getter and setter
}

class PersonComparator implements Comparator<Person> {
    @Override
    public int compare(Person p1, Person p2) {
       return (p1.getMathScore() > p2.getMathScore()) ? -1 :
               ((p1.getMathScore() == p2.getMathScore()) ? 0 : 1);
    }
}