Connecting onboard into Socket

Can somebody show me how to correctly plug onboard (the keyboard) into the Socket?
I have:

I add a socket into expander:

        let expander: Expander = builder.object("expander").expect("Couldn't get expander");
        let socket: Socket = gtk::Socket::new();

I spawn onboard:

let io: Result<(Pid, i32, i32, i32), Error> =
        glib::spawn_async_with_pipes(working_directory, &argv, &envp, flags, child_setup);

I create io channel for communication between onboard and the widget:

let channel = glib_sys::g_io_channel_unix_new(pid.2);

Then I create watch for watching the events from onboard:

 let socket_ptr: *mut std::ffi::c_void =
                    &mut socket as *mut _ as *mut std::ffi::c_void;
                let user_data: [gpointer; 1] = [socket_ptr];
                glib_sys::g_io_add_watch(channel, condition, Some(watch_out_channel), socket_ptr);

And finally I add the window id to the socket:

unsafe extern "C" fn watch_out_channel(
    channel: *mut GIOChannel,
    condition: u32,
    socket: gpointer,
) -> gboolean {
    let mut raw: *mut c_char = std::ptr::null_mut();
    let size: *mut GType = std::ptr::null_mut();
    let terminator_pos: *mut GType = std::ptr::null_mut();
    let ptr_error = std::ptr::null_mut();
    glib_sys::g_io_channel_read_line(channel, &mut raw, size, terminator_pos, ptr_error);
    if !raw.is_null() {
        println!("{}", *raw);
        let window_number = (*raw).into();
        gtk_sys::gtk_socket_add_id(socket as *mut GtkSocket, window_number);

But unfortunately I'm getting crash. It stops debugging in the file called @g_type_check_instance_is_a

When I highlight the socket variable I see info that there is a invalid enum value.

Thanks for any help with this