Avoid fatal error: Uncaught Error: Call to a member function get_id() on bool

[mircobabini]

$_product can be false. This is because the ->get_product() has these return types: \WC_Product|false. Then we should check if $_product instanceof \WC_Product or continue;

Cases:

• Order with an empty line
• Plugins which permit to buy (in WC cart) items (CPT) which are NOT extending the \WC_Product class.
• Order with a raw line_item