Estou tentando medir o tamanho de um rótulo GTK:

#include <gtk/gtk.h>

static void map_event(GtkWidget *window, gpointer lab) {
    g_print( "In the callback..\n" );
    GtkWidget *label = GTK_WIDGET(lab);
    g_print( "Everything is ok..\n" );
}

static void activate (GtkApplication* app, gpointer user_data)
{
    GtkWidget *window = gtk_application_window_new (app);
    gtk_window_set_title (GTK_WINDOW (window), "Window1");
    gtk_window_set_default_size (GTK_WINDOW (window), 200, 280);
    GtkWidget *grid = gtk_grid_new ();
    gtk_container_add (GTK_CONTAINER (window), grid);
    GtkWidget *label = gtk_label_new("Hello world!");
    gtk_grid_attach(GTK_GRID (grid), label, 0,0,1,1);
    g_signal_connect (window, "map-event", G_CALLBACK(map_event), label);
    gtk_widget_show_all (window);
}

int main (int argc, char **argv) {
    GtkApplication *app = gtk_application_new (
        "org.gtk.example", G_APPLICATION_FLAGS_NONE );
    g_signal_connect( app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    g_object_unref (app);

    return status;
}

Isso fornece saída:

In the callback..
Segmentation fault (core dumped)

Se eu comentar a linha:

GtkWidget *label = GTK_WIDGET(lab);

não há falha de segmentação, o rótulo aparece e a saída é:

In the callback..
Everything is ok..

O que estou perdendo aqui?