check null safety

lib/domain/entities/rate.dart

@@ -1,4 +1,3 @@
-
 import 'package:fcs/domain/entities/discount_by_weight.dart';
 
 import 'cargo_type.dart';
@@ -11,26 +10,25 @@ class Rate {
   double diffDiscountWeight;
   double diffWeightRate;
 
-  List<CargoType> cargoTypes;
-  List<CargoType> customDuties;
-  List<DiscountByWeight> discountByWeights;
+  List<CargoType> cargoTypes = [];
+  List<CargoType> customDuties = [];
+  List<DiscountByWeight> discountByWeights = [];
 
   DiscountByWeight getDiscountByWeight(double weight) {
     discountByWeights.sort((d1, d2) => d2.weight.compareTo(d1.weight));
-    return discountByWeights.firstWhere((e) => e.weight < weight,
-        orElse: () => null);
+    return discountByWeights.firstWhere((e) => e.weight < weight);
   }
 
-  CargoType get defaultCargoType => cargoTypes == null
+  CargoType? get defaultCargoType => cargoTypes == null
       ? null
       : cargoTypes.firstWhere((e) => e.name == "General");
 
   Rate(
-      {this.deliveryFee,
-      this.freeDeliveryWeight,
-      this.volumetricRatio,
-      this.diffDiscountWeight,
-      this.diffWeightRate});
+      {this.deliveryFee = 0,
+      this.freeDeliveryWeight = 0,
+      this.volumetricRatio = 0,
+      this.diffDiscountWeight = 0,
+      this.diffWeightRate = 0});
 
   factory Rate.fromMap(Map<String, dynamic> map) {
     return Rate(