Check for WP Error to prevent fatal error

[mntzrr]

Handle situation where $payson_order is treated as an array, but it is a WP_Error object.

I am not entirely sure about whether to show the notice. Perhaps it should also be logged, as updating order reference is necessary to identify the order in the merchant portal.

[NiklasHogefjord]

Task linked: CU-3n5jkxe Fatal error on merchant reference update