get_stock_quantity fails on products that are not valid

Plugin crashes when you have a deleted product in cart

PHP Fatal error: Uncaught Error: Call to a member function get_stock_quantity() on bool in wp-content/plugins/wc-smart-cod-pro/public/class-wc-smart-cod-public.php:1500

The fix would be to check wc_get_product if exists:

        foreach ( $cart_products as $product ) {
            $_product = wc_get_product( $product );
            $qty = !$_product ? 0 : $_product->get_stock_quantity();
            $stock    = $qty;