NullPointerException in problem 3.3

[damluar]

When you try to pop, directly after creation, you will get Null Pointer Exception in int v = last.pop(), since the list of stacks is empty.

public int pop() {
    Stack last = getLastStack();
    int v = last.pop();
    if (last.size == 0) {
        stacks.remove(stacks.size() - 1);
    }
    return v;
}