error

D:\testing\rb_test>python main_test.py
Traceback (most recent call last):
  File "main_test.py", line 8, in <module>
    admin.delete_test_user(driver, actions)
  File "D:\testing\rb_test\admin_actions.py", line 35, in delete_test_user
    actions.move_to_element(user_menu).click(admin_button).perform()
  File "C:\Python27\lib\site-packages\selenium\webdriver\common\action_chains.py
", line 72, in perform
    action()
  File "C:\Python27\lib\site-packages\selenium\webdriver\common\action_chains.py
", line 217, in <lambda>
    self._driver.execute(Command.MOVE_TO, {'element': to_element.id}))
  File "C:\Python27\lib\site-packages\selenium\webdriver\remote\webdriver.py", l
ine 193, in execute
    self.error_handler.check_response(response)
  File "C:\Python27\lib\site-packages\selenium\webdriver\remote\errorhandler.py"
, line 181, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.StaleElementReferenceException: Message: Element not
found in the cache - perhaps the page has changed since it was looked up
Stacktrace:
    at fxdriver.cache.getElementAt (resource://fxdriver/modules/web-element-cach
e.js:9348)
    at Utils.getElementAt (file:///c:/users/mapapp/appdata/local/temp/tmphbvfy_/
extensions/fxdriver@googlecode.com/components/driver-component.js:8942)
    at fxdriver.events.buildCoordinates (file:///c:/users/mapapp/appdata/local/t
emp/tmphbvfy_/extensions/fxdriver@googlecode.com/components/driver-component.js:
10165)
    at FirefoxDriver.prototype.mouseMoveTo (file:///c:/users/mapapp/appdata/loca
l/temp/tmphbvfy_/extensions/fxdriver@googlecode.com/components/driver-component.
js:10974)
    at DelayedCommand.prototype.executeInternal_/h (file:///c:/users/mapapp/appd
ata/local/temp/tmphbvfy_/extensions/fxdriver@googlecode.com/components/command-p
rocessor.js:12643)
    at DelayedCommand.prototype.executeInternal_ (file:///c:/users/mapapp/appdat
a/local/temp/tmphbvfy_/extensions/fxdriver@googlecode.com/components/command-pro
cessor.js:12648)
    at DelayedCommand.prototype.execute/< (file:///c:/users/mapapp/appdata/local
/temp/tmphbvfy_/extensions/fxdriver@googlecode.com/components/command-processor.
js:12590)