fix: add guard checks when destroying window

[matheus-inacio]

Description

This PR fixes a race condition that occurs when a window is being destroyed (closed).

Signals like notify::size, size-changed, notify::appears-focused or workspace-changed can sometimes fire after the underlying Meta.Window object has been cleared, but before the actor is fully disconnected. This results in the extension trying to access properties on a null window object, causing the following error:

JS ERROR: TypeError: can't access property "get_client_type", win is null
getRoundedCornersEffect@file:///home/matheus/.local/share/gnome-shell/extensions/rounded-window-corners@fxgn/manager/utils.js:50:5
refreshRoundedCorners@file:///home/matheus/.local/share/gnome-shell/extensions/rounded-window-corners@fxgn/manager/event_handlers.js:164:43
applyEffectTo/<@file:///home/matheus/.local/share/gnome-shell/extensions/rounded-window-corners@fxgn/manager/event_manager.js:119:18
_destroyWindowDone@resource:///org/gnome/shell/ui/windowManager.js:1590:21
onStopped@resource:///org/gnome/shell/ui/windowManager.js:1558:39
_makeEaseCallback/<@resource:///org/gnome/shell/ui/environment.js:66:22
_easeActor/<@resource:///org/gnome/shell/ui/environment.js:161:64
@resource:///org/gnome/shell/ui/init.js:21:20

PS: In the error log above, the crash specifically happened during the size-changed signal.

Changes

I've added null checks for actor.metaWindow within the signal callbacks in event_manager.ts. This ensures that the handlers only run if the window still actively exists.

[flexagoon]

Is it maybe possible to just move this check into the connect method to avoid repeating it everywhere? And then you could also add a comment above it explaining why it's needed

[flexagoon]

Never mind, moving it into connect would probably be very had to do in TypeScript because the actor variable is defined in different places. LGTM then, thanks!