Fix crashing on macos when setting theme to none

src/platform_impl/macos/window.rs

@@ -345,15 +345,15 @@ pub(super) fn set_ns_theme(theme: Option<Theme>) {
     let app: id = msg_send![app_class, sharedApplication];
     let has_theme: BOOL = msg_send![app, respondsToSelector: sel!(effectiveAppearance)];
     if has_theme == YES {
-      let name = if let Some(theme) = theme {
-        NSString::alloc(nil).init_str(match theme {
+      let appearance = if let Some(theme) = theme {
+        let name = NSString::alloc(nil).init_str(match theme {
           Theme::Dark => "NSAppearanceNameDarkAqua",
           Theme::Light => "NSAppearanceNameAqua",
-        })
+        });
+        msg_send![class!(NSAppearance), appearanceNamed: name]
       } else {
         nil
       };
-      let appearance: id = msg_send![class!(NSAppearance), appearanceNamed: name];
       let _: () = msg_send![app, setAppearance: appearance];
     }
   }