尝试指向其他实例时发生NullPointerException

Question

在制作和检查combining.degree时，我尝试访问已编辑的多边形节点，但是却收到NullPointerException，如何解决此问题？我想使用在while循环中编辑过的poly，而不是在while循环之外设为null的poly。这是

    public static Node multiply(Node poly1, Node poly2) {
    Node poly3=poly1;
    Node poly4=poly2;
    Node poly=null;
    while (poly3!=null)
    {
        while(poly4!=null)
        {
            float x;
            int y;
            x=poly3.term.coeff*poly4.term.coeff;
            y=poly3.term.degree*poly4.term.degree;
            poly=new Node(x,y,poly);
            poly4=poly4.next;
        }
        poly4=poly2;
        poly3=poly3.next;
    }
    
    Node finalpoly=null;
    int i=poly.term.degree;
    float csum=0;
    Node combining = poly;
    while (i>=combining.term.degree)
    {
        while (combining!=null)
        {
            if (i==combining.term.degree)
            {
                csum+=combining.term.coeff;
            }
            combining=combining.next;
        }
        if (csum!=0)
        {
            finalpoly = new Node(csum, i, finalpoly);
        }
        i--;
    }