The function counts the number of visible children and tries to allocate returned widths and heights based on that count.  But if the # of visible children is 0, the g_new() call return NULL because it was asked to allocate no memory, and callers of gtk_button_box_child_requisition() like gtk_button_box_size_allocate() which try to use the returned allocated arrays crash.

  while (children)
    {
      ...
      if (gtk_widget_get_visible (child))
        {
          nchildren += 1;
          ...
        }
    }

  *widths = g_new (gint, nchildren);
  *heights = g_new (gint, nchildren);

etc, *widths and *heights will be NULL on return which doesn't work so well :(