ClassNotFoundException error on order status change [1.7.6.8]

[Mirooon]

Hey everyone!

I have a problem with 500 err when I'm trying to change status of order from e.g. "waiting for payment" to "package ready to send". I turned on debug mode and this is what log says:

Symfony\Component\Debug\Exception\
ClassNotFoundException

in override/classes/Product.php (line 46)

if (class_exists('Product')) {
	Product::$definition['fields']['xmlupdate'] = array(			<-- line 46
		'type' => 2, 'shop' => true, 'validate' => 'isBool'
	);
}

PHP version is 7.2.2
Presta version: 1.7.6.8

I cleared cache.

Do you have any ideas, how to fix it?

Thank you guys!