add: if ( ! navigation_tree_view ) { return 0; }
context: gint gsb_gui_navigation_get_current_report ( void ){    GtkTreeSelection *selection;    GtkTreeIter iter;    gint page;    selection = gtk_tree_view_get_selection (GTK_TREE_VIEW (navigation_tree_view));    if (! gtk_tree_selection_get_selected (selection, NULL, &iter))	return 0;    gtk_tree_model_get ( GTK_TREE_MODEL (navigation_model),			 &iter,			 NAVIGATION_PAGE, &page,			 -1);        if ( page == GSB_REPORTS_PAGE)    {	gint report_number;	gtk_tree_model_get (GTK_TREE_MODEL(navigation_model), &iter, NAVIGATION_REPORT, &report_number, -1);	return report_number;    }    return 0;}