When user closes customer sheet without choosing new card error occurs

[JSBmanD]

Describe the bug
The error being thrown if the user closes customer sheet without choosing another card, not already selected.

To Reproduce
Steps to reproduce the behavior:

1. Open customer sheet
2. Add card
3. Confirm and close
4. Open sheet again
5. Just close

Expected behavior
CustomerSheetResult or null is returned

Actual behavior

Stacktrace

type 'Null' is not a subtype of type 'String' in type cast
    type 'Null' is not a subtype of type 'String' in type cast
    #0      _$$StripeErrorGenericImplFromJson (package:stripe_platform_interface/src/models/errors.g.dart:12:32)
    #1      new _$StripeErrorGenericImpl.fromJson (package:stripe_platform_interface/src/models/errors.freezed.dart:123:7)
    #2      _$StripeErrorFromJson (package:stripe_platform_interface/src/models/errors.freezed.dart:18:10)
    #3      new StripeError.fromJson (package:stripe_platform_interface/src/models/errors.dart:26:7)
    #4      _$$CustomerSheetResultImplFromJson (package:stripe_platform_interface/src/models/customer_sheet.g.dart:115:13)
    #5      new _$CustomerSheetResultImpl.fromJson (package:stripe_platform_interface/src/models/customer_sheet.freezed.dart:973:7)
    #6      _$CustomerSheetResultFromJson (package:stripe_platform_interface/src/models/customer_sheet.freezed.dart:804:31)
    #7      new CustomerSheetResult.fromJson (package:stripe_platform_interface/src/models/customer_sheet.dart:96:7)
    #8      MethodChannelStripe._parseCustomerSheetResult (package:stripe_platform_interface/src/method_channel_stripe.dart:357:36)
    #9      MethodChannelStripe.presentCustomerSheet (package:stripe_platform_interface/src/method_channel_stripe.dart:277:12)
    <asynchronous suspension>
    #10     _SetupFuturePaymentScreenState._handleSavePress (package:client_app/src/feature/order/widget/test_stripe.dart:140:22)
    <asynchronous suspension>
    #11     _LoadingButtonState._loadFuture (package:client_app/src/feature/order/widget/widgets/loading_button.dart:50:7)
    <asynchronous suspension>

Sample

``` Future _handleSavePress() async { if (_card == null) { return; } try { // 1. Create setup intent on backend final stripeConfig = await _createSetupIntentOnBackend(_email);

  // 2. Gather customer billing information (ex. email)
  const billingDetails = BillingDetails(
    name: 'Test User',
    email: '[email protected]',
    phone: '+48888000888',
    address: Address(
      city: 'Houston',
      country: 'US',
      line1: '1459  Circle Drive',
      line2: '',
      state: 'Texas',
      postalCode: '77063',
    ),
  ); // mo/ mocked data for tests

  // 3. Confirm setup intent

  await Stripe.instance.initCustomerSheet(
    customerSheetInitParams: CustomerSheetInitParams(
      setupIntentClientSecret: stripeConfig.clientSecret,
      customerId: stripeConfig.customerId,
      customerEphemeralKeySecret: stripeConfig.ephemeralKeySecret,
    ),
  );

  final result = await Stripe.instance.presentCustomerSheet();

  print(result);
} catch (error) {
  ScaffoldMessenger.of(context)
      .showSnackBar(SnackBar(content: Text('Error code: $error')));
  rethrow;
}

}

</details>


**Smartphone / tablet**
 - Device: Iphone 15 simulator
 - OS: ios 17.2
 - Package version: 10.0.0
 - Flutter version 3.16.0

[remonh87]

created a pull request for fixing it