The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability at line 40 is:

1. Lack of null pointer check: The code directly compares the 'rt' pointer with 'net->ipv6.ip6_null_entry' without first verifying if 'rt' is NULL.

2. Potential NULL return from pol_lookup_func: The 'rt' pointer is assigned the result of pol_lookup_func() on line 38. If this function can return NULL, and there's no check for NULL before using 'rt', it could lead to a null pointer dereference.

3. Insufficient error handling: The code doesn't appear to handle the case where pol_lookup_func() might fail and return NULL, leading to unsafe use of the 'rt' pointer.

4. Assumption of non-NULL return: The code assumes that pol_lookup_func() always returns a valid pointer, which may not be true in all cases.

To fix this vulnerability, a null check should be added immediately after the pol_lookup_func() call:

```c
rt = pol_lookup_func(lookup, ...);
if (rt == NULL) {
    // Handle error case
    return ERROR_CODE;
}
if (rt != net->ipv6.ip6_null_entry) {
    // Proceed with the existing logic
}
```

This check ensures that 'rt' is not NULL before it's dereferenced or compared, preventing the potential null pointer dereference.