Fix get related order IDs method

changelog.txt

@@ -1,5 +1,8 @@
 *** WooCommerce Subscriptions Core Changelog ***
 
+= 7.1.2 - xxxx-xx-xx =
+* Fix - Add check to prevent fatal error when retrieving related order IDs.
+
 = 7.1.1 - 2024-05-09 =
 * Fix - Resolved an issue that caused "WC_DateTime could not be converted to int" warnings to occur on non-hpos sites while editing a subscription.
 

includes/class-wc-subscription.php

@@ -2065,7 +2065,10 @@ protected function get_related_order_ids( $order_type = 'any' ) {
 			$relation_types = ( 'any' === $order_type ) ? array( 'renewal', 'resubscribe', 'switch' ) : array( $order_type );
 
 			foreach ( $relation_types as $relation_type ) {
-				$related_order_ids = array_merge( $related_order_ids, WCS_Related_Order_Store::instance()->get_related_order_ids( $this, $relation_type ) );
+				$related_orders_for_relation_type = WCS_Related_Order_Store::instance()->get_related_order_ids( $this, $relation_type );
+				if ( is_array( $related_orders_for_relation_type ) ) {
+					$related_order_ids = array_merge( $related_order_ids, $related_orders_for_relation_type );
+				}
 			}
 		}