NullPointException Java

Question

I wrote this code to merge two Lists (NodoLista is a single element of the list), but I get the NullPointerExcpetion at line 34 of Main. Can you help me?

public static void main(String[] args) {
    NodoLista a= new NodoLista(4,new NodoLista(5,new NodoLista(6,null)));
    NodoLista b= new NodoLista(7,new NodoLista(8, new NodoLista(9,null)));
    a=fusioneOrdinata(a,b);
    while(a!=null){
        System.out.println(a.info);
        a=a.next;
    }
}

static NodoLista fusioneOrdinata(NodoLista x, NodoLista y){
NodoLista pp=null;
NodoLista a=x;
NodoLista b=y;
while(b!=null){
    while(a!=null || b.info>a.info){
        pp=a;
        a=a.next;
    }
    if(b.info<=a.info){
        pp.next=b;
        b.next=a;
    }
    b=b.next;
}
return a;}

public class NodoLista {
int info; //OBJECT= Classe da cui derivano tutte le altre, contenitore di oggetti (non di tipi primitivi)
NodoLista next; // RICORSIVA

NodoLista(int info, NodoLista next){
    this.info=info;
    this.next=next;
}}