Is a GtkWidget a decendant of a Gobject?

[LWATCDR <lwatcdr gmail com>]

I am making a simple c++ wrapper for GTK for my own use.
When this line of code 
gobject=(GObject *)gtk_window_new(GTK_WINDOW_TOPLEVEL);
executes I get these messages.

testgtk:11206): GLib-GObject-WARNING **: invalid (NULL) pointer instance

(testgtk:11206): GLib-GObject-CRITICAL **: g_signal_connect_data: assertion `G_TYPE_CHECK_INSTANCE (instance)' failed

everything seems to work fine but they worry me. What is up?