On macOS, fix tabGroup misuse

The property is marked as `Weak`, however we used strong `Id`.

Links: https://github.com/alacritty/alacritty/issues/7249

src/platform_impl/macos/appkit/window.rs

@@ -218,7 +218,7 @@ extern_methods!(
         pub(crate) fn tabbingIdentifier(&self) -> Id<NSString>;
 
         #[method_id(tabGroup)]
-        pub(crate) fn tabGroup(&self) -> Id<NSWindowTabGroup>;
+        pub(crate) fn tabGroup(&self) -> Option<Id<NSWindowTabGroup>>;
 
         #[method(isDocumentEdited)]
         pub(crate) fn isDocumentEdited(&self) -> bool;