Error

Hi, we receive this error:
Fatal error: Uncaught Error: Call to a member function __isset() on null in /home/xxxx/public_html/xxxx.it/wp-content/plugins/wc-fields-factory/includes/wcff-injector.php:266 Stack trace: #0 /home/xxxx/public_html/xxxx.it/wp-content/plugins/wc-fields-factory/includes/wcff-injector.php(88): Wcff_FieldsInjector->render_admin_fields() #1 //home/xxxx/public_html/xxxx.it/wp-content/plugins/wc-fields-factory/includes/wcff-product-fields.php(119): Wcff_FieldsInjector->inject(‘woocommerce_bef…’) #2 /home/xxxx/public_html/xxxx.it/wp-includes/class-wp-hook.php(286): Wcff_ProductFields->fields_injector(”) #3 /home/xxxx/public_html/xxxx.it/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array) #4 /home/xxxx/public_html/xxxx.it/wp-includes/plugin.php(465): WP_Hook->do_action(Array) #5 /home/xxxx/public_html/xxxx.it/wp-content/plugins/woocommerce/templates/content-product.php(42): in /home/xxxx/public_html/xxxx.it/wp-content/plugins/wc-fields-factory/includes/wcff-injector.php on line 266