我正在尝试抓取Google App Store，但是在运行脚本时，出现以下错误：“不支持的URL方案”：该方案没有可用的处理程序。 代码如下：

    # -*- coding: utf-8 -*-
    import scrapy

    # from scrapy.spiders import CrawlSpider,Rule
    # from scrapy.linkextractors import LinkExtractor
    # from html.parser import HTMLParser as SGMLParser
    from gp.items import GpItem


class GoogleSpider(scrapy.Spider):
    # print("HELLO STARTING")
    name = 'google'
    allowed_domains = ['play.google.com']
    start_urls = ['https://play.google.com/store/apps/']

    '''
    rules = [
        Rule(LinkExtractor(allow=("https://play\.google\.com/store/apps/details",)),callback='parse_app',follow=True),]
    '''

    def parse(self,response):
        print("CALLING PARSE")
        selector = scrapy.Selector(response)

        # print(response.body)

        urls = selector.xpath('//a[@class="LkLjZd ScJHi U8Ww7d xjAeve nmZKrb  id-track-click "]/@href').extract()

        link_flag = 0

        links = []
        for link in urls:
            print("LINK" + str(link))
            links.append(link)

        for each in urls:
            # print(links[link_flag])
            yield scrapy.Request(links[link_flag],callback=self.parse_next,dont_filter=True)
            link_flag += 1

    def parse_next(self,response):
        selector = scrapy.Selector(response)

        # print(response)
        app_urls = selector.xpath('//div[@class="details"]/a[@class="title"]/@href').extract()
        # print(app_urls)

        urls = []
        for url in app_urls:
            url = "http://play.google.com" + url
            # print(url)
            urls.append(url)

        link_flag = 0
        for each in app_urls:
            yield scrapy.Request(urls[link_flag],callback=self.parse_app,dont_filter=True)
            link_flag += 1

    def parse_app(self,response):
        item = GpItem()
        item['app_url'] = response.url
        item['app_name'] = response.xpath('//div[@itemprop="name"]').xpath('text()').extract()
        item['app_icon'] = response.xpath('//img[@itemprop="image"]/@src')
        # item['app_developer'] = response.xpath('//')
        # print(response.text)
        yield item

错误信息如下：

[scrapy.downloadermiddlewares.robotstxt] ERROR: Error downloading <GET :///robots.txt>: Unsupported URL scheme '': no handler available for that scheme
Traceback (most recent call last):
  File "/anaconda3/lib/python3.7/site-packages/twisted/internet/defer.py",line 1416,in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/anaconda3/lib/python3.7/site-packages/twisted/python/failure.py",line 512,in throwExceptionIntoGenerator
    return g.throw(self.type,self.value,self.tb)
  File "/anaconda3/lib/python3.7/site-packages/scrapy/core/downloader/middleware.py",line 43,in process_request
    defer.returnValue((yield download_func(request=request,spider=spider)))
  File "/anaconda3/lib/python3.7/site-packages/scrapy/utils/defer.py",line 45,in mustbe_deferred
    result = f(*args,**kw)
  File "/anaconda3/lib/python3.7/site-packages/scrapy/core/downloader/handlers/__init__.py",line 70,in download_request
    (scheme,self._notconfigured[scheme]))
scrapy.exceptions.NotSupported: Unsupported URL scheme '': no handler available for that scheme
2019-11-15 08:49:14 [scrapy.core.scraper] ERROR: Error downloading <GET /store/apps/collection/cluster?clp=ogoKCAEqAggBUgIIAQ%3D%3D:S:ANO1ljJG6Aw&gsr=Cg2iCgoIASoCCAFSAggB:S:ANO1ljLKNqE>
scrapy.exceptions.NotSupported: Unsupported URL scheme '': no handler available for that scheme

我试图改为将extract（）放到extract_first（）中，它将显示另一个错误，例如'raise ValueError（'在请求url中缺少方案：％s'％self._url）'。我是新手，请有人帮我解决这个问题吗？