NullPointerException when removing element from stack

I'm trying to implement a Java stack structure, but when I try to remove an element I get the following error:

> Exception in thread "main" java.lang.NullPointerException
    at br.com.magnoliamedeiros.pilhafila.LinkedList.remove(LinkedList.java:17)
    at br.com.magnoliamedeiros.pilhafila.Pilha.pop(Pilha.java:13)
    at br.com.magnoliamedeiros.pilhafila.Program.main(Program.java:22)

Any idea what might be happening? (error lines are marked in code snippets)

My program:

//...
Pilha<String> pilha = new Pilha<String>();          
//...
pilha.push("Márcia");
pilha.pop(); // LINHA #22

Class Pilha :

public class Pilha<T> extends LinkedList<T>{        
    public void push(T valor){
        add(valor);
    }

    public T pop() throws Exception{
        if(isEmpty()){
            throw new Exception("Pilha vazia!");
        }
        return this.remove(0); // LINHA #13
    }
}

Class Node :

public class Node<T> {
    Node<T> proximo;
    Node<T> anterior;
    T valor;

    public void setProximo(Node<T> proximo){
        this.proximo = proximo;
    }

    public Node<T> getProximo(){
        return proximo;
    }

    public void setAnterior(Node<T> anterior){
        this.anterior = anterior;
    }

    public Node<T> getAnterior(){
        return anterior;
    }

    public void setValor(T valor){
        this.valor = valor;
    }

    public T getValor(){
        return valor;
    }
}

Class LinkedList :

public class LinkedList<T> implements java.util.List<T>{
    Node<T> inicio;
    Node<T> fim;
    int size = 0;

    @Override
    public T remove(int index) {
        if(index==0){
                inicio.getAnterior().setProximo(inicio.getProximo()); // LINHA #17
                inicio.getProximo().setAnterior(inicio.getAnterior());
                inicio.setAnterior(null);
                inicio.setProximo(null);
                return inicio.getValor();
        }else{
            int i = 1;
            while(i!=this.size()){
                i++;
                inicio.setProximo(inicio.getProximo());

                if(i == index){
                    inicio.getAnterior().setProximo(inicio.getProximo());
                    inicio.getProximo().setAnterior(inicio.getAnterior());
                    inicio.setAnterior(null);
                    inicio.setProximo(null);
                    return inicio.getValor();
                }
            }
        }
        return null;
    }


    public boolean add(T e){
        Node<T> newNode = new Node<T>();
        newNode.setValor(e);

        if(inicio == null){
            inicio = newNode;
        }
        if(fim == null){
            fim = newNode;
        }else{
            fim.setProximo(newNode);
            newNode.setAnterior(fim);
            fim = newNode;
        }
        size++;
        return true;
    }

    @Override
    public int size() {
        return size;
    }


    @Override
    public boolean isEmpty() {
        return size==0;
    }

}