Flutter widget tests do not pass when run together

I am trying write to some tests for a simple credit card form widget.

Each test passes when run individually but the second one fails and throws the following exception when I run both of them.

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following IndexError was thrown running a test:
RangeError (index): Index out of range: no indices are valid: 0

I tried different approaches such as resetting testedWidget to null in tearDown hook but nothing worked as intended.

Here is the test file I wrote:

void main() {
  final validCardNumber = '1234123412341234';
  final validExpirationDate = '11/25';
  final invalidExpirationDate = '32/25';
  final validCvv = '123';
  Widget testedWidget;

  setUp(() {
    testedWidget = testableAddCardScreen(AddCardFormScreen());
  });

  testWidgets('Form is submittable if inputs are valid', (
    WidgetTester tester,
  ) async {
    await tester.pumpWidget(testedWidget);
    await tester.pumpAndSettle();

    final inputs = find.byType(TextInput);

    await tester.enterText(inputs.at(0), validCardNumber);
    await tester.enterText(inputs.at(1), validExpirationDate);
    await tester.enterText(inputs.at(2), validCvv);
    await tester.pumpAndSettle();

    final button = evaluateWidget<Button>(find.byType(Button));

    expect(button.isDisabled, false);
  });

  testWidgets('Form is not submittable if inputs are invalid', (
    WidgetTester tester,
  ) async {
    await tester.pumpWidget(testedWidget);
    await tester.pumpAndSettle();

    final inputs = find.byType(TextInput);

    await tester.enterText(inputs.at(0), validCardNumber);
    await tester.enterText(inputs.at(1), invalidExpirationDate);
    await tester.enterText(inputs.at(2), validCvv);
    await tester.pumpAndSettle();

    final button = evaluateWidget<Button>(find.byType(Button));

    expect(button.isDisabled, true);
  });
}

I also use some utils functions to make widgets testable.

Widget testableAddCardScreen(Widget child) {
  return testableLocalizedWidgetFactory(
    ChangeNotifierProvider(
      create: (_) => AddCardProvider(
        apiClientService: MockApiClientService(),
        successCallback: () {},
        failureCallback: () {},
      ),
      child: child,
    ),
  );
}