Undefined companionWindowIds property of window on updateWindow

[barmintor]

See #3437

I'm working on a plugin that updates the window from an interaction in the sidebar. When the update is processed, a selector (which is somewhat obscured by the intermediating software) triggers a similar error - namely, that companionWindowIds is undefined on the window (which does exist), and thus the call to companionWindowIds.map(...) fails.

This is happening on a fork of M3, but as far as I can tell the relevant code is unchanged in M4. I'm suspicious that there's a race condition here. It might also be something in my plugin, but I don't want to lose this thread.