Button activation behavior can call "show a modal dialog" with not-connected dialog

[zcorpan]

What is the issue with the HTML Standard?

The introduction of command/commandfor in #9841 specifies new activation behavior steps for button which can call "show a modal dialog" without checking if that dialog is connected, in which case it will throw an exception. It's not immediately clear to me what should happen in that case (should the exception be propagated to click()?).

Demo: https://software.hixie.ch/utilities/js/live-dom-viewer/saved/13512

As far as I can tell the other exception-throwing cases in "show a modal dialog" are already accounted for.

cc @keithamus