check null safety

lib/domain/entities/payment.dart

@@ -1,11 +1,11 @@
 import 'package:cloud_firestore/cloud_firestore.dart';
 
 class Payment {
-  String id;
-  String invoiceID;
-  DateTime paymentDate;
-  String paymentReceiptURL;
-  String status;
+  String? id;
+  String? invoiceID;
+  DateTime? paymentDate;
+  String? paymentReceiptURL;
+  String? status;
   double amount;
 
   Payment(
@@ -14,13 +14,13 @@ class Payment {
       this.paymentDate,
       this.paymentReceiptURL,
       this.status,
-      this.amount});
+      this.amount = 0});
 
   factory Payment.fromMap(Map<String, dynamic> map, String id) {
     var _paymentDate = (map['payment_date'] as Timestamp);
     return Payment(
       id: id,
-      paymentDate: _paymentDate?.toDate(),
+      paymentDate: _paymentDate.toDate(),
       paymentReceiptURL: map['payment_receipt_url'],
       status: map['status'],
       amount: map['amount']?.toDouble() ?? 0,
@@ -31,7 +31,7 @@ class Payment {
     return {
       "id": id,
       "invoice_id": invoiceID,
-      'payment_date': paymentDate?.toUtc()?.toIso8601String(),
+      'payment_date': paymentDate?.toUtc().toIso8601String(),
       'payment_receipt_url': paymentReceiptURL,
       'status': status,
       'amount': amount,
@@ -39,7 +39,7 @@ class Payment {
   }
 
   Payment clone() {
-    return Payment.fromMap(toMap(), this.id);
+    return Payment.fromMap(toMap(), this.id!);
   }
 
   @override