Selecting popup without title and clicking button inside it with geb

I have a popup without title and I want to detect the window using any other criteria (for example a div that wraps the whole popup) and click a button inside it.

I've tried the following:

withWindow({ $("div", class:"main.msgAlerta" }) {

                $('#close').click()

But I always get a org.openqa.selenium.ElementNotVisibleException at te #close action. Am I failing at detecting the window or is the error related to the button?