This is a downstream copy of https://gitlab.gnome.org/GNOME/gnome-boxes/-/issues/722 for blocker/FE purposes. As described there:

In GNOME Boxes 41-rc1, the "Welcome Tutorial" displayed when the app first runs is entirely blank, and cannot be closed with Esc as used to be the case. You can only close it by clicking the X. When run from a console, the following errors are shown:

    (gnome-boxes:43242): Gtk-CRITICAL **: 10:25:33.839: Error building template class 'BoxesWelcomeTutorial' for an instance of type 'BoxesWelcomeTutorial': .:2:1119 Invalid object type 'BoxesWelcomeTutorialPage'

    (gnome-boxes:43242): Gtk-CRITICAL **: 10:25:33.840: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed

    (gnome-boxes:43242): Handy-CRITICAL **: 10:25:33.840: hdy_carousel_get_position: assertion 'HDY_IS_CAROUSEL (self)' failed

    (gnome-boxes:43242): Gtk-CRITICAL **: 10:25:33.840: gtk_widget_set_opacity: assertion 'GTK_IS_WIDGET (widget)' failed

    (gnome-boxes:43242): Gtk-CRITICAL **: 10:25:33.840: gtk_widget_set_opacity: assertion 'GTK_IS_WIDGET (widget)' failed

    (gnome-boxes:43242): Boxes-CRITICAL **: 10:25:33.840: boxes_welcome_tutorial_page_get_color: assertion 'self != NULL' failed

    (gnome-boxes:43242): Boxes-CRITICAL **: 10:25:33.840: boxes_welcome_tutorial_page_get_color: assertion 'self != NULL' failed

Proposing as a Beta FE - this doesn't stop the app working, but it does give a poor first impression (and is obviously bad for someone who actually needs the tutorial).