在python末尾插入元素时，单链表无限循环

class ListNode:
    def __init__(self, val=None):
        self.val = val
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None
        self.tail = None
    
    def insert(self, val):
        new_node = ListNode(val)
        if self.head is None:
            self.head = new_node
            self.tail = new_node
        else:
            self.tail.next = new_node
            self.tail = new_node

my_list = LinkedList()
my_list.insert(1)
my_list.insert(2)
my_list.insert(3)