keyboard: Fix warnings on destroyed actors

Trying to hide a page gets hard when the page was previouly destroyed.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1281

js/ui/keyboard.js

@@ -1477,6 +1477,7 @@ var Keyboard = class Keyboard {
             c.destroy();
         });
 
+        this._currentPage = null;
         this._groups = {};
         this._onGroupChanged();
     }