Getting Null Pointer Error In Double Linked List Project

I am currently making a double linked list class in my compsci class. i was absent for a few days and i need to make an add method for the class. when i go to compile test code i made i get a null pointer error on line 36. This is the node class i wrote and the double linked list class i wrote.

node class

public class Node{
// Two references and Data
Node prev;
Node next;
String data;
public Node(){
prev = null;
next = null;

[Code] ....

This is the class i need working with ( in the doublelinkedlist class)

public void add(Node n){
if(isEmpty()){
first = n;
return;
}
Node temp = first;
while(temp != null){
temp = temp.getNext();
}
temp.setNext(n);
temp.setPrev(temp);
}