File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/webdriver.py", line 934, in save_screenshot
    return self.get_screenshot_as_file(filename)
  File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/webdriver.py", line 911, in get_screenshot_as_file
    png = self.get_screenshot_as_png()
  File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/webdriver.py", line 943, in get_screenshot_as_png
    return base64.b64decode(self.get_screenshot_as_base64().encode('ascii'))
  File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/webdriver.py", line 953, in get_screenshot_as_base64
    return self.execute(Command.SCREENSHOT)['value']
  File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/webdriver.py", line 311, in execute
    self.error_handler.check_response(response)
  File "/usr/local/lib/python2.7/dist-packages/selenium/webdriver/remote/errorhandler.py", line 237, in check_response
    raise exception_class(message, screen, stacktrace)
WebDriverException: Message: TypeError: rootNode is null