Fix segfault when dropping MacWindow (#9267)

This avoids calling `window.setDelegate(nil)` when the window was
already closed.

Release Notes:

- Fixed a segfault that could show up when closing windows.

Co-authored-by: Antonio <antonio@zed.dev>
754df9356dThorsten Ball committed on 3/13/2024, 9:41:25 AM· committed by GitHubparent3c28282
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