Fix after rebase

backend/flea-market-manager-service/src/main/java/cz/fi/muni/pa165/flea/market/manager/service/StandService.java

@@ -93,8 +93,11 @@ public class StandService extends DomainServiceImpl<Stand> {
                 .map(ItemCategory::valueOf)
                 .collect(Collectors.toSet()));
 
-        Optional.of(standUpdateDTO.getSeller())
-                        .ifPresent(seller -> stand.setSeller(userService.findById(seller.getId())));
+        if (standUpdateDTO.getSeller() != null) {
+            stand.setSeller(userService.findById(standUpdateDTO.getSeller().getId()));
+        } else {
+            stand.setSeller(null);
+        }
         stand.setStandType(standTypeServiceService.findById(standUpdateDTO.getStandType().getId()));
         stand.setEvent(marketEventDao.findById(standUpdateDTO.getEventId()));