This function has a returm type but doesn't ends with return statement

Я получаю предупреждение "This function has a return type of 'FutureOr<List>', but doesn't end with a return statement"

Мой код

    _readTaskList().then((dynamic value) {
      if (value != null) {
        final List<Task> tasks = <Task>[];
        final List<Task> _tasks = value as List<Task>;
        tasks.forEach((dynamic element) {
          _tasks.add(_convertFromJsonToTask(element as Map<String, dynamic>));
        });
        return tasks;
      } else {
        return null;
      }
    });
  }