gtk_{color,font}_dialog_button_new(): NULL gives critical message
The descriptions of gtk_color_dialog_button_new() and gtk_font_dialog_button_new() say that You can pass NULL to this function, but that's not possible because of
g_return_val_if_fail (GTK_IS_FONT_DIALOG (dialog), NULL);
Critical messages are printed:
(gtk4-demo:17243): Gtk-CRITICAL **: 12:03:55.109: gtk_color_dialog_button_new: assertion 'GTK_IS_COLOR_DIALOG (dialog)' failed
(gtk4-demo:17243): Gtk-CRITICAL **: 12:03:55.110: gtk_font_dialog_button_new: assertion 'GTK_IS_FONT_DIALOG (dialog)' failed
gtk_color_dialog_button_set_dialog() contains the same call to g_return_val_if_fail(), while gtk_font_dialog_button_set_dialog() accepts NULL:
g_return_if_fail (dialog == NULL || GTK_IS_FONT_DIALOG (dialog));