linux/x11: Mark windows as destroyed after destroy request (#12892)

Turns out we still get FocusOut and UnmapNotify events after the window
has been destroyed, which resulted in error messages popping up because
we can't find the window anymore that we want to mark as unfocused.



Release Notes:

- N/A

crates/gpui/src/platform/linux/x11/client.rs

@@ -458,6 +458,7 @@ impl X11Client {
         state
             .windows
             .get(&win)
+            .filter(|window_reference| !window_reference.window.state.borrow().destroyed)
             .map(|window_reference| window_reference.window.clone())
     }