无法通过Modal对话框出现（Selenium :: WebDriver :: Error :: UnhandledAlertError）

I'm using watir-webdriver and i'm having trouble with a confirmation popup. I click on a 'Sell' button and a confirmation popup appears. I can't seem to figure out how to come up with the step to click 'OK' on the popup. Any help would be much appreciated.I'm using watir-webdriver and i'm having troubl