Why is there an "attribute error" inside the condition of the while loop? but not the if statement?

I am writing an algorithm to merge two sorted list from leetcode. I noticed that the conditions in while loops were throwing

Attribute Error: 'Nontype' object has no attribute 'val'

After I deleted all the '.val' methods inside the while conditionals, it compiled normally. Then why is it that the conditionals in the if statements have no problem of "attribute error"?

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

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        result = []
        while l1.val != None and l2.val != None:
            if l1.val >= l2.val:
                result.append(l2.val)
                l2 = l2.next
            else:
                result.append(l1.val)
                l1 = l1.next
        if l1.val != None:
            while l1.val != None:
                result.append(l1.val)
                l1 = l1.next
        else:
            while l2.val != None:
                result.append(l2.val)
                l2 = l2.next
        return result