[gtkmm] How to make a toplevel window visible with a popup window

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I'm trying to make a toplevel window visible for a popup window.

I've a popup window that gets all the visible area of the desktop and need 
to  create another toplevel window when a certain event occurs.

I've tried to use the Gtk::Window::set_transient_for but as the main window 
is a derived class of Gtk::Window when a use:

this->toplevel_window->set_transient_for(popup_window);

the compiler says that it can't  find a function like 
toplevel_window->set_transient_for(popup_window*&) and that the candidates 
are Gtk::Window::set_transient_for(Gtk::Window*&).

I've tried, also, Gdk::Window::set_transient_for(Gdk::Window) but it only 
works if the popup_window is a toplevel window.

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