public void setReturnShipNode(ReturnChannelResponse response, String returnShipNode) {
        log.debug("Entering setReturnShipNode method");

        setReturnShipNodeForOrderItemList(response.getOrderItemList(), returnShipNode);
        setReturnShipNodeForOrderChannels(response.getOrderChannels(), returnShipNode);

        log.debug("Exit setReturnShipNode method");
    }

    private void setReturnShipNodeForOrderItemList(List<OrderItem> orderItemList, String returnShipNode) {
        for (OrderItem orderItem : orderItemList) {
            if (ObjectUtils.allNotNull(orderItem.getItemDetails(), orderItem.getItemDetails().getAddress())) {
                String childClientId = orderItem.getItemDetails().getChildClientId();
                String postalCode = orderItem.getItemDetails().getAddress().getPostalCode();

                if (ObjectUtils.isNotEmpty(childClientId) && "0".equals(childClientId)) {
                    orderItem.getItemDetails().getAddress().setReturnShipNode(returnShipNode);
                    log.debug("Setting returnShipNode for OrderItem with Address: {}", orderItem.getItemDetails().getAddress());
                } else if (ObjectUtils.isNotEmpty(postalCode) && postalCode.startsWith("XX")) {
                    orderItem.getItemDetails().getAddress().setReturnShipNode(returnShipNode);
                    log.debug("Setting returnShipNode for OrderItem with Address: {}", orderItem.getItemDetails().getAddress());
                }
            }
        }
    }

    private void setReturnShipNodeForOrderChannels(List<OrderChannel> orderChannels, String returnShipNode) {
        for (OrderChannel orderChannel : orderChannels) {
            if (ObjectUtils.allNotNull(orderChannel.getAddress())) {
                String childClientId = orderChannel.getAddress().getChildClientId();
                String postalCode = orderChannel.getAddress().getPostalCode();

                if (ObjectUtils.isNotEmpty(childClientId) && "0".equals(childClientId)) {
                    orderChannel.getAddress().setReturnShipNode(returnShipNode);
                    log.debug("Setting returnShipNode for OrderChannel with Address: {}", orderChannel.getAddress());
                } else if (ObjectUtils.isNotEmpty(postalCode) && postalCode.startsWith("XX")) {
                    orderChannel.getAddress().setReturnShipNode(returnShipNode);
                    log.debug("Setting returnShipNode for OrderChannel with Address: {}", orderChannel.getAddress());
                }
            }
        }
    }
}

orderItem.getItemDetails() is wrong.....use below correct method calling  and update the above code

public void setReturnShipNode(ReturnChannelResponse response, String returnShipNode) {
    log.debug("Entering setReturnShipNode method");
    setReturnShipNodeForOrderItemList(response.getOrderItemList(), returnShipNode);
    setReturnShipNodeForOrderChannels(response.getOrderChannels(), returnShipNode);
    log.debug("Exit setReturnShipNode method");
  }

  private void setReturnShipNodeForOrderItemList(List<OrderItem> orderItemList, String returnShipNode) {
    for (OrderItem orderItem : orderItemList) {
      log.debug("Setting returnShipNode for LineItemChannels of OrderItem: {}", orderItem.getLineItemChannels());
      setReturnShipNodeForLineItemChannels(orderItem.getLineItemChannels(), returnShipNode);
    }
  }

  private void setReturnShipNodeForLineItemChannels(List<LineItemChannel> lineItemChannels, String returnShipNode) {
    for (LineItemChannel lineItemChannel : lineItemChannels) {
      log.debug("Setting returnShipNode for LineItemChannel with address childClientId: {}", lineItemChannel.getAddress().getChildClientId());
      if (lineItemChannel != null && lineItemChannel.getAddress() != null && lineItemChannel.getAddress().getChildClientId() != null && lineItemChannel.getAddress().getChildClientId().equals("0")) {
        lineItemChannel.getAddress().setReturnShipNode(returnShipNode);
      }
    }
  }

  private void setReturnShipNodeForOrderChannels(List<OrderChannel> orderChannels, String returnShipNode) {
    for (OrderChannel orderChannel : orderChannels) {
      if (orderChannel != null && orderChannel.getAddress() != null && orderChannel.getAddress().getChildClientId() != null && orderChannel.getAddress().getChildClientId().equals("0")) {
        log.debug("Setting returnShipNode for OrderChannel with address childClientId: {}", orderChannel.getAddress().getChildClientId());
        orderChannel.getAddress().setReturnShipNode(returnShipNode);
      }
    }
  }