Why is my FlutterDriver null when i call it from a classes' method?

Question

I am beginning to learn automated tests with flutter, since i want to use the flutter_driver to take a screenshot on a failed expect i created a class that does a try of the test and catches TestFailure by doing a screenshot and rethrowing the failure.

I am using the default flutter app (the one with the counter and the button to increase it).

import 'package:flutter_driver/flutter_driver.dart';
import 'package:test/test.dart';
import 'dart:io';

class CustomTest {
  Function testDetails;
  String testName;
  FlutterDriver? driverr;
  CustomTest(this.testDetails, this.testName, this.driverr);
  void runTest() {
    test(testName, () async {
      try {
        await testDetails();
      } on TestFailure catch (_) {
        await takeScreenshot();
        rethrow;
      }
    });
  }

  Future<File> takeScreenshot() async {
    final List<int> pixels = await driverr!.screenshot();
    final File file = File('test_driver/results/${testName}_failure.png');
    await file.writeAsBytes(pixels);
    return file;
  }
}

void main() {
  group('Counter App', () {
    FlutterDriver? driver;
    setUpAll(() async {
      driver = await FlutterDriver.connect();
    });

    tearDownAll(() async {
      driver?.close();
    });

    final counterTextFinder = find.byValueKey('counter');
    final buttonFinder = find.byValueKey('increment');

    CustomTest test1 = CustomTest(() async {
      expect(await driver?.getText(counterTextFinder), "1");
    }, "Starts at 0", driver);
    test1.runTest();

    CustomTest test2 = CustomTest(() async {
      await driver?.tap(buttonFinder);
      expect(await driver?.getText(counterTextFinder), "2");
    }, "Increments the counter", driver);
    test2.runTest();
  });
}

However, when i run my tests (which i modified to fail to test the screenshot) i get a null exception because the driver is null.

I tried extracting the code from the takeScreenshot() method and putting it on the test description itself, there it runs without issues.