這是我的 getBefore 方法 public Node getBefore() {    return getBeforeHelper(root, this.data);}public Node getBeforeHelper(Node node, K key) {    Node current = null;    if(node != null) {        if(node.data == key) {            if(node.left != null) {                current = node.left;                while(current.right != null) {                    current = current.right;                }                System.out.println(current.get());                return current;            }        }        else if(lessThan.test(key, node.data)) {            return getBeforeHelper(node.left, key);        }        else if(lessThan.test(node.data, key)) {            return getBeforeHelper(node.right, key);        }    }    else {        return null;    }    return current;}這是它未能通過的 Junit 測(cè)試@Testpublic void beforeBST() {BinarySearchTree<Integer> bst = new BinarySearchTree<>((Integer x, Integer y) -> x < y);assertTrue(bst.isEmpty());int[] a = new int[] { 12, 4, 18, 5, 11, 8, 15, 9, 17, 20, 3, 13, 19, 2, 14, 7, 6, 10, 1, 16 };int n = a.length;for (Integer key : a)   bst.insert(key);assertNull(bst.search(1).getBefore());for (int i = 2; i <= n; i++) {    System.out.println(bst.search(2).getBefore());    assertTrue(i - 1 == bst.search(i).getBefore().get());}}它在最后一個(gè) for 循環(huán)中到達(dá) assertTrue，然后因空指針異常而失敗。為什么會(huì)拋出空指針？