[GtkGLExt] Calling constructor of parent widget - runtime error

[Philipp Klaus Krause <pkk spth de>]

In the gtkglextmm examples the derived widget's constructor never calls
the parent's constructor. When I add the call like

Display::Display(BaseObjectType *da, Glib::RefPtr<Gnome::Glade::Xml>&
refGlade) : Gtk::DrawingArea(da)
{
...

I get a runtime error:
GtkGLExt-CRITICAL **: gtk_widget_set_gl_capability:
assertion `!GTK_WIDGET_REALIZED (widget)' failed

However gtkmm docs recommend to always call the parent's constructor.
Not doing so will screw up signal handlers.

Philipp