It print "invalid uninstantiatable type" when closing the window on Linux

[topurce]

Just run the example on linux and click the close button, then console output below messages:

(bitsdojo_window_example:227071): GLib-GObject-CRITICAL **: 23:59:55.515: invalid uninstantiatable type '' in cast to 'GtkWidget'

(bitsdojo_window_example:227071): GLib-GObject-CRITICAL **: 23:59:55.515: invalid uninstantiatable type '' in cast to 'GtkWindow'

(bitsdojo_window_example:227071): Gtk-CRITICAL **: 23:59:55.515: gtk_window_set_application: assertion 'GTK_IS_WINDOW (window)' failed