title: Not able to locate element on a modal pop up window in Selenium
body: <p>I am automating a webpage using Selenium Webdriver. I am not able to click a button a modal pop up window using simple element locator method.</p>
  
  <p>Example:</p>
  
  <ul>
  <li>open www.walmart.com </li>
  <li>enter tv in the search box. </li>
  <li>select some tv and click "Add to Cart"</li>
  <li>Now a pop up window comes where "Checkout" button is located. I need to click on this "checkout" button.</li>
  </ul>
  
  <p>I tried switchTo() windowhandle, I tried switchTo() frame but nothing worked.</p>