Browser pop-up windows are recognized as instances of Browser.
When the popup window is active, it is seen as Browser and the original browser is seen as Browser 2. In order to make DefaultBaseState() close the pop-up window instead of the original browser, add the following line to the end of the test case:
Browser2.SetActive()
This is the standard way of ensuring that the pop-up becomes Browser2 and is closed by DefaultBaseState().