null safety

lib/domain/entities/invoice.dart

@@ -28,7 +28,7 @@ class Invoice {
   List<CustomDuty> customDuties;
   List<Carton> cartons;
   List<CargoType> cargoTypes;
-  List<Shipment> shipments;
+  List<Shipment?>? shipments;
   List<Payment> payments;
   Discount? discount;
   PaymentMethod? paymentMethod;
@@ -85,9 +85,9 @@ class Invoice {
   }
 
   double getHandlingFee() {
-    return shipments
-        .where((sh) => sh.isSelected)
-        .fold(0, (p, s) => p + (s.handlingFee - s.paidHandlingFee));
+    return shipments!
+        .where((sh) => sh!.isSelected)
+        .fold(0, (p, s) => p + (s!.handlingFee - s.paidHandlingFee));
   }
 
   double getTotalBalance(Rate rate) {
@@ -176,7 +176,7 @@ class Invoice {
     List _cargoTypes = cargoTypes.map((c) => c.toMap()).toList();
     List _customDuties = customDuties.map((c) => c.toMap()).toList();
     List _cartons = cartons.map((c) => c.toMap()).toList();
-    List _shipments = shipments.map((s) => s.toMap()).toList();
+    List _shipments = shipments!.map((s) => s!.toMap()).toList();
     return {
       "id": id,
       "invoice_date": invoiceDate?.toUtc().toIso8601String(),

lib/domain/entities/package.dart

@@ -36,7 +36,7 @@ class Package {
   DeliveryAddress? deliveryAddress;
 
   //for packages in processing
-  List<File> photoFiles;
+  List<File?> photoFiles;
 
   int get amount => rate != null && weight != null ? rate * weight : 0;