check null safety

lib/domain/vo/shipment_status.dart

@@ -3,17 +3,21 @@ import 'package:cloud_firestore/cloud_firestore.dart';
 class ShipmentStatus {
   String status;
   DateTime date;
-  bool done;
-  String staffId;
-  String staffName;
+  bool? done;
+  String? staffId;
+  String? staffName;
   ShipmentStatus(
-      {this.status, this.date, this.done, this.staffId, this.staffName});
+      {required this.status,
+      required this.date,
+      this.done,
+      this.staffId,
+      this.staffName});
 
   factory ShipmentStatus.fromMap(Map<String, dynamic> map) {
     var _date = (map['date'] as Timestamp);
     return ShipmentStatus(
         status: map['status'],
-        date: _date == null ? null : _date.toDate(),
+        date: _date.toDate(),
         done: map['done'],
         staffId: map['staff_id'],
         staffName: map['staff_name']);