PHP warnings

Reported by @Leonidas in[ WordPress support forum](https://wordpress.org/support/topic/php-warning-429/).

- [x] Undefined array key "plugins" in `class-addonify-wishlist.php` on line 309.
- [x] foreach() argument must be of type array|object, null given in `class-addonify-wishlist.php` on line 309.