main: Don't set stage key focus to stage itself

Moving the key focus to the stage should be done by unsetting
the key focus, not by setting it to the stage itself.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3633>

js/ui/main.js

@@ -737,7 +737,8 @@ export function pushModal(actor, params = {}) {
     });
 
     actionMode = newActionMode;
-    global.stage.set_key_focus(actor);
+    const newFocus = actor === global.stage ? null : actor;
+    global.stage.set_key_focus(newFocus);
     return grab;
 }