Java 尝试合并链接列表时获取空指针异常

我正在尝试合并两个链表。我编写了以下代码从ListB中选择节点，并在适当的位置将其链接到listA。然而，我得到空指针异常，无法找出原因。。请帮忙

  Node sortedMerge(Node headA, Node headB) {

     if(headB.data<=headA.data){
         Node temp = headB.next;
         headB.next=headA;
         headA=headB;
        if (temp == null) return headA;
         else return sortedMerge(headA, temp);
     }
     else{
         Node headAtemp = headA;
         while(headA.next.data <headB.data && headA.next!=null){
             headA = headA.next;
         }
         Node next = headA.next;
         Node temp = headB.next;
         //changing links
         headA.next= headB;
         headB.next = next;
         if (temp == null) return headAtemp;
         else return sortedMerge(headAtemp,temp);
     }
   }

节点分拣机（节点头A、节点头B）{
如果（标题b.数据更改顺序，使其从左向右读取

while(headA.next!=null && headA.next.data <headB.data ){

while（headA.next！=null&&headA.next.data