gnome-fonts-viewer shows a lot of warnings in console.
I see the following warnings in console when using gnome-font-viewer
gnome-font-viewer
(gnome-font-viewer:7071): GLib-GObject-WARNING **: 03:53:44.267: invalid cast from 'FontViewApplication' to 'SushiFontWidget'
(gnome-font-viewer:7071): GLib-GObject-WARNING **: 03:53:44.369: invalid cast from 'FontViewApplication' to 'SushiFontWidget'
(gnome-font-viewer:7071): GLib-GObject-WARNING **: 03:56:18.900: invalid cast from 'FontViewApplication' to 'SushiFontWidget'
[hussam@hades ~]$ gnome-font-viewer
(gnome-font-viewer:8251): GLib-GObject-WARNING **: 03:56:40.472: invalid cast from 'FontViewApplication' to 'SushiFontWidget'
(gnome-font-viewer:8251): GLib-GObject-WARNING **: 03:56:40.570: invalid cast from 'FontViewApplication' to 'SushiFontWidget'
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.469: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.469: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.470: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.471: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed
(gnome-font-viewer:8251): Gtk-CRITICAL **: 04:01:09.471: gtk_tree_model_filter_real_unref_node: assertion 'elt->ref_count > 0' failed