Attribute「AttributeError： 'MemoryUsage' object has no attribute 'tasks'」

    def __init__(self, crawler):
        if not crawler.settings.getbool('MEMUSAGE_ENABLED'):
            raise NotConfigured
        try:
            # stdlib's resource module is only available on unix platforms.
            self.resource = import_module('resource')
        except ImportError:
            raise NotConfigured
        self.tasks = [] #没定义，那就加上吧
        self.crawler = crawler
        self.warned = False
        self.notify_mails = crawler.settings.getlist('MEMUSAGE_NOTIFY_MAIL')
        self.limit = crawler.settings.getint('MEMUSAGE_LIMIT_MB')*1024*1024
        self.warning = crawler.settings.getint('MEMUSAGE_WARNING_MB')*1024*1024
        self.check_interval = crawler.settings.getfloat('MEMUSAGE_CHECK_INTERVAL_SECONDS')
        self.mail = MailSender.from_settings(crawler.settings)
        crawler.signals.connect(self.engine_started, signal=signals.engine_started)
        crawler.signals.connect(self.engine_stopped, signal=signals.engine_stopped)

「self.tasks = []＃未定義の場合、追加してください」