cleanup code

lib/pages/shipment/shipment_editor.dart

@@ -279,19 +279,19 @@ class _ShipmentEditorState extends State<ShipmentEditor> {
     }).toList();
   }
 
-  _addBox(Carton box) {
+  _addBox(Carton? box) {
     if (box == null) return;
     box.cartonType = carton_from_shipments;
     _shipment!.boxes.add(box);
     setState(() {});
   }
 
-  _saveBox(Carton box) {
+  _saveBox(Carton? box) {
     if (box == null) return;
     setState(() {});
   }
 
-  _removeBox(Carton box) {
+  _removeBox(Carton? box) {
     if (box == null) return;
     _shipment!.boxes.remove(box);
     setState(() {});