Call to a member function validate()

Hello there
When I go cart page, there is an error sometimes. it is not always if there is error I just refresh the page then error will be gone. Can you have a look at error and explain me what might be a reason it?

Fatal error: Uncaught Error: Call to a member function validate() on null in /Applications/XAMPP/xamppfiles/htdocs/futureshop.2.4.4/vqmod/vqcache/vq2-pages_ajax_checkout_customer.inc.php:249 Stack trace: #0 /Applications/XAMPP/xamppfiles/htdocs/futureshop.2.4.4/includes/library/lib_route.inc.php(125): include() #1 /Applications/XAMPP/xamppfiles/htdocs/futureshop.2.4.4/index.php(40): route::process() #2 {main} thrown in /Applications/XAMPP/xamppfiles/htdocs/futureshop.2.4.4/vqmod/vqcache/vq2-pages_ajax_checkout_customer.inc.php on line 249

I checked the file that error mentioned line, there is nothing wrong with it.

The error suggests that when you are saving the details, $mod_customer fails to initiate the customer modules. Probably because the lines is removed or similar.
https://github.com/litecart/litecart/blob/2.5.3/public_html/pages/ajax/checkout_customer.inc.php#L51-L53

it is not removed in file. it mentioned this code on line.

'error' => $order->validate(isset($shipping) ? $shipping : ''),

  $box_checkout_customer->snippets = [
    'account_exists' => $account_exists,
    'subscribed_to_newsletter' => $subscribed_to_newsletter,
    'error' => $order->validate(isset($shipping) ? $shipping : ''),
  ];
  echo $box_checkout_customer->stitch('views/box_checkout_customer');```

is it might be a reason because of null value of $shipping variable ?

It doesn't look like mine:

  $box_checkout_customer->snippets = [
    'account_exists' => $account_exists,
    'subscribed_to_newsletter' => $subscribed_to_newsletter,
  ];
  echo $box_checkout_customer->stitch('views/box_checkout_customer');```

Is this a super old version?

it is 2.4.4 v or older.

so, it is not necessary right ?

No, you can toss that line.
2.5.x is a version great of fun, I recommend upgrading.