check null safety

lib/domain/entities/invoice.dart

@@ -10,15 +10,15 @@ import 'package:fcs/domain/entities/rate.dart';
 import 'package:fcs/domain/entities/shipment.dart';
 
 class Invoice {
-  String id;
-  String invoiceNumber;
-  DateTime invoiceDate;
-  String fcsShipmentID;
-  String userID;
-  String fcsID;
-  String userName;
-  String phoneNumber;
-  String status;
+  String? id;
+  String? invoiceNumber;
+  DateTime? invoiceDate;
+  String? fcsShipmentID;
+  String? userID;
+  String? fcsID;
+  String? userName;
+  String? phoneNumber;
+  String? status;
 
   double handlingFee;
   double deliveryFee;
@@ -30,9 +30,9 @@ class Invoice {
   List<CargoType> cargoTypes;
   List<Shipment> shipments;
   List<Payment> payments;
-  Discount discount;
-  PaymentMethod paymentMethod;
-  String invoiceURL;
+  Discount? discount;
+  PaymentMethod? paymentMethod;
+  String? invoiceURL;
 
   List<CargoType> getCargoTypes(Rate rate) {
     if (cargoTypes != null) return cargoTypes;
@@ -40,7 +40,7 @@ class Invoice {
     List<CargoType> _cargoTypes = [];
     double totalCalWeight = 0;
     cartons.forEach((carton) {
-      if (carton.isChecked) {
+      if (carton.isChecked ?? false) {
         var _cartonsTypes =
             carton.getCargoTypeForCalWeight(rate.volumetricRatio);
         _cartonsTypes.forEach((ct) {
@@ -68,15 +68,15 @@ class Invoice {
 
   double getTotal(Rate rate) {
     List<CargoType> cargoTypes = getCargoTypes(rate);
-    var total = cargoTypes.fold(0.0, (p, c) => c.calAmount + p);
+    double total = cargoTypes.fold(0.0, (p, c) => c.calAmount + p);
     return total;
   }
 
-  double get balance => (amount ?? 0) - (paidAmount ?? 0);
+  double get balance => amount - paidAmount;
 
   double getNetAmount(Rate rate) {
     List<CargoType> cargoTypes = getCargoTypes(rate);
-    var total = cargoTypes.fold(0.0, (p, c) => c.calAmount + p);
+    double total = cargoTypes.fold(0.0, (p, c) => c.calAmount + p);
     total += getCustomFee();
     total += getDeliveryFee();
     total += getHandlingFee();
@@ -85,13 +85,13 @@ class Invoice {
   }
 
   double getHandlingFee() {
-    return shipments?.where((sh) => sh.isSelected ?? false)?.fold(0, (p, s) {
-      return p + (s?.handlingFee ?? 0) - (s?.paidHandlingFee ?? 0);
-    });
+    return shipments
+        .where((sh) => sh.isSelected)
+        .fold(0, (p, s) => p + (s.handlingFee - s.paidHandlingFee));
   }
 
   double getTotalBalance(Rate rate) {
-    return getNetAmount(rate) - (paidAmount ?? 0);
+    return getNetAmount(rate) - paidAmount;
   }
 
   double getCustomFee() {
@@ -102,7 +102,7 @@ class Invoice {
     return deliveryFee == null ? 0 : deliveryFee;
   }
 
-  double getDiscount() => discount == null ? 0 : discount.amount;
+  double getDiscount() => discount == null ? 0 : discount!.amount;
 
   Invoice(
       {this.id,
@@ -111,19 +111,19 @@ class Invoice {
       this.fcsID,
       this.userName,
       this.phoneNumber,
-      this.amount,
-      this.paidAmount,
+      this.amount = 0,
+      this.paidAmount = 0,
       this.discount,
       this.status,
-      this.customDuties,
-      this.cartons,
-      this.cargoTypes,
-      this.handlingFee,
-      this.deliveryFee,
+      this.customDuties = const [],
+      this.cartons = const [],
+      this.cargoTypes = const [],
+      this.handlingFee = 0,
+      this.deliveryFee = 0,
       this.fcsShipmentID,
-      this.shipments,
+      this.shipments = const [],
       this.invoiceURL,
-      this.payments,
+      this.payments = const [],
       this.paymentMethod});
 
   factory Invoice.fromMap(Map<String, dynamic> map, String docID) {
@@ -153,7 +153,7 @@ class Invoice {
     return Invoice(
       id: docID,
       invoiceNumber: map['invoice_number'],
-      invoiceDate: invd?.toDate(),
+      invoiceDate: invd.toDate(),
       userName: map['user_name'],
       fcsID: map['fcs_id'],
       phoneNumber: map['phone_number'],
@@ -174,12 +174,12 @@ class Invoice {
 
   Map<String, dynamic> toMap() {
     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 _customDuties = customDuties.map((c) => c.toMap()).toList();
+    List _cartons = cartons.map((c) => c.toMap()).toList();
+    List _shipments = shipments.map((s) => s.toMap()).toList();
     return {
       "id": id,
-      "invoice_date": invoiceDate?.toUtc()?.toIso8601String(),
+      "invoice_date": invoiceDate?.toUtc().toIso8601String(),
       "user_id": userID,
       "user_name": userName,
       "invoice_number": invoiceNumber,