title: ReactorNotRestartable error in while loop with scrapy
body: <p>I get <code>twisted.internet.error.ReactorNotRestartable</code> error when I execute following code:</p>
  
  <pre><code>from time import sleep
  from scrapy import signals
  from scrapy.crawler import CrawlerProcess
  from scrapy.utils.project import get_project_settings
  from scrapy.xlib.pydispatch import dispatcher
  
  result = None
  
  def set_result(item):
      result = item
  
  while True:
      process = CrawlerProcess(get_project_settings())
      dispatcher.connect(set_result, signals.item_scraped)
  
      process.crawl('my_spider')
      process.start()
  
      if result:
          break
      sleep(3)
  </code></pre>
  
  <p>For the first time it works, then I get error. I create <code>process</code> variable each time, so what's the problem?</p>