windowAttentionHandler: Don't destroy on open

The activateWindow() call is expected to focus the window, which
already destroys the source. If we then destroy it again explicitly,
we get another "invalid access" warning.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/430

js/ui/windowAttentionHandler.js

@@ -101,6 +101,5 @@ var Source = class WindowAttentionSource extends MessageTray.Source {
 
     open() {
         Main.activateWindow(this._window);
-        this.destroy();
     }
 };