Another exception was thrown: NoSuchMethodError: The getter 'zipCode' was called on null

estou fazendo uma tela de calcular frete, o codigo ta todo " funcionando" porem na hora de rodar o app e chegar na tela de calular me apresenta esse Erro

Segue alguns Códigos.

import 'package:Flutter2/src/address.dart';
import 'package:Flutter2/src/cart_manager.dart';
import 'package:Flutter2/src/custom_icon_button.dart';
import 'package:brasil_fields/formatter/cep_input_formatter.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:provider/provider.dart';

class CepInputField extends StatefulWidget {
  const CepInputField(this.address);

  final Address address;

  @override
  _CepInputFieldState createState() => _CepInputFieldState();
}

class _CepInputFieldState extends State<CepInputField> {
  final TextEditingController cepController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    final cartManager = context.watch<CartManager>();
    final primaryColor = Theme.of(context).primaryColor;

    if (widget.address.zipCode == null)
      return Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          TextFormField(
            enabled: !cartManager.loading,
            controller: cepController,
            decoration: const InputDecoration(
                isDense: true, labelText: 'CEP', hintText: '12.345-678'),
            inputFormatters: [
              WhitelistingTextInputFormatter.digitsOnly,
              CepInputFormatter(),
            ],
            keyboardType: TextInputType.number,
            validator: (cep) {
              if (cep.isEmpty)
                return 'Campo obrigatório';
              else if (cep.length != 10) return 'CEP Inválido';
              return null;
            },
          ),
          if (cartManager.loading)
            LinearProgressIndicator(
              valueColor: AlwaysStoppedAnimation(primaryColor),
              backgroundColor: Colors.transparent,
            ),
          RaisedButton(
            onPressed: !cartManager.loading
                ? () async {
                    if (Form.of(context).validate()) {
                      try {
                        await context
                            .read<CartManager>()
                            .getAddress(cepController.text);
                      } catch (e) {
                        Scaffold.of(context).showSnackBar(SnackBar(
                          content: Text('$e'),
                          backgroundColor: Colors.red,
                        ));
                      }
                    }
                  }
                : null,
            textColor: Colors.white,
            color: primaryColor,
            disabledColor: primaryColor.withAlpha(100),
            child: const Text('Buscar CEP'),
          ),
        ],
      );
    else
      return Padding(
        padding: const EdgeInsets.symmetric(vertical: 4),
        child: Row(
          children: <Widget>[
            Expanded(
              child: Text(
                'CEP: ${widget.address.zipCode}',
                style:
                    TextStyle(color: primaryColor, fontWeight: FontWeight.w600),
              ),
            ),
            CustomIconButton(
              iconData: Icons.edit,
              color: primaryColor,
              size: 20,
              onTap: () {
                context.read<CartManager>().removeAddress();
              },
            ),
          ],
        ),
      );
  }
}

    class Address {
  Address(
      {this.street,
      this.number,
      this.complement,
      this.district,
      this.zipCode,
      this.city,
      this.state,
      this.lat,
      this.long});

  String street;
  String number;
  String complement;
  String district;
  String zipCode;
  String city;
  String state;

  double lat;
  double long;

  Address.fromMap(Map<String, dynamic> map) {
    street = map['street'] as String;
    number = map['number'] as String;
    complement = map['complement'] as String;
    district = map['district'] as String;
    zipCode = map['zipCode'] as String;
    city = map['city'] as String;
    state = map['state'] as String;
    lat = map['lat'] as double;
    long = map['long'] as double;
  }

  Map<String, dynamic> toMap() {
    return {
      'street': street,
      'number': number,
      'complement': complement,
      'district': district,
      'zipCode': zipCode,
      'city': city,
      'state': state,
      'lat': lat,
      'long': long,
    };
  }
}

Agradeço a todos