Fix panic in window open (#4148)

Fix a panic caused by toggleFullScreen on one window untoggling
full-screen on
another.

I managed to reproduce this only once, which makes me sad, but this
change
should fix it regardless.

https://zed-industries.slack.com/archives/C04S6T1T7TQ/p1705631681238979

Release Notes:

- Fixed an occasional panic when opening a new fullscreen window.