linter错误是由对then(...)的调用引起的，dart linter急切地将其解析为then<String>而不是then<String?>。
您可以显式指定类型以解决此行为：

Future<List<String>> testFunc() {
  List<Future<String?>> result = List.empty(growable: true);

  for(int i = 0; i < 10; i++) {
    result.add(
      Future.delayed(Duration(seconds: i), () {
        if(i % 2 == 0) { return i; }
        else { throw Error(); }
      })
      .then<String?>((value) { // <- Change here!
        return "got from future : $value";
      })
      .catchError((error) {
        return null; // No more linter warning
      })
    );
  }

  // then, wait for all futures, and remove the ones that crashed
  return Future.wait(result).then((listOfNullable) => listOfNullable.where((element) => element != null).map((e) => e!).toList());
}

也就是说，提供给Future<T>.catchError的回调必须返回T或Future<T>（也称为FutureOr<T>）。
由于您在Future.then<R>返回的Future<R>上调用catchError，因此错误回调必须返回FutureOr<R>。您需要：

.then((value) {
    return "got from future : $value"; 
  })

由于没有为Future.then<R>提供显式类型，R是从callback的返回类型（不可为空的String. As jraufeisen explained）中推断出来的，因此可以通过显式指定R（而不是允许它被推断出来）来修复此问题。
或者，我强烈建议使用async-await和try-catch，而不是使用Future.then和Future.catchError。这样做可以避免这些问题（或者至少使它们更容易推理）。在您的情况下，helper函数会使转换更容易：

Future<List<String>> testFunc() {
  // create a result list
  List<Future<String?>> result = List.empty(growable: true);

  Future<String?> helper(int i) async {
    try {
      // create a future that may fail
      var value = await Future.delayed(Duration(seconds: i), () {
        if (i % 2 == 0) {
          return i;
        } else {
          throw Error();
        }
      });
      // if the future succeed, return the value
      return "got from future : $value";
    } on Error {
      // otherwise, returns "null" as we are expecting a future returning a nullable value
      return null;
    }
  }

  // put 10 future in the results
  for (int i = 0; i < 10; i++) {
    result.add(helper(i));
  }

  // then, wait for all futures, and remove the ones that crashed
  return Future.wait(result).then((listOfNullable) => listOfNullable
      .where((element) => element != null)
      .map((e) => e!)
      .toList());
}

使用.map代替.where来过滤掉null s，使用collection-for来避免创建一个空的List并使其增长，并删除剩余的Future.then：

Future<List<String>> testFunc() async {
  Future<String?> helper(int i) async {
    try {
      // create a future that may fail
      var value = await Future.delayed(Duration(seconds: i), () {
        if (i % 2 == 0) {
          return i;
        } else {
          throw Error();
        }
      });
      // if the future succeed, return the value
      return "got from future : $value";
    } on Error {
      // otherwise, returns "null" as we are expecting a future returning a nullable value
      return null;
    }
  }

  // then, wait for all futures, and remove the ones that crashed
  var result = await Future.wait([
    for (var i = 0; i < 10; i++) helper(i),
  ]);
  return result.whereType<String>().toList();
}