Error on checkout page if using existing domain while ordering new hosting

[Mukesh Kumar]

Hello Team

After WHMCS 8.8 version update,  I am getting below error at the time of checkout if i am using my existing domain and update my nameservers.  So can any one check this issue and suggest me how to resolve it. 

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TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in /home/troonle/public_html/members/vendor/whmcs/whmcs-foundation/lib/OrderForm.php:0
Stack trace:
#0 /home/troonle/public_html/members/vendor/whmcs/whmcs-foundation/lib/OrderForm.php(0): count(NULL)
#1 /home/troonle/public_html/members/includes/orderfunctions.php(0): WHMCS\OrderForm->getNumItemsInCart(Object(WHMCS\User\Client))
#2 /home/troonle/public_html/members/cart.php(0): calcCartTotals(Object(WHMCS\User\Client), true, false)
#3 {main}

Thanks