Steps to reproduce:
1. Compile/link testgtk.c (from gtk test suite) under Windows using MSVC6.
2. Start program; select "Display and Screen"; 
3. Hit cancel: works OK
4. Select "Display and Screen" again;
5. Hit down-arrow on GtkCombo widget twice 
  (to cause the pop-down to display;
6. Hit cancel on the "display and Screen" window: the program will crash.

---- or ----

compile/link/run simple test program as shown in "other information" below.



Stack trace:
(I don't have Windows debug vesions of GTk and don't know how to create same).

LIBGDK-WIN32-2.0-0! 6b088390()
LIBGDK-WIN32-2.0-0! 6b05c929()
LIBGDK-WIN32-2.0-0! 6b05ca2a()
LIBGTK-WIN32-2.0-0! 606c32e4()
LIBGOBJECT-2.0-0! 62743995()
LIBGOBJECT-2.0-0! 62756dc6()
LIBGOBJECT-2.0-0! 62757f3e()
LIBGOBJECT-2.0-0! 627581b6()
LIBGTK-WIN32-2.0-0! 606bf78c()
LIBGTK-WIN32-2.0-0! 606c64e0()
LIBGOBJECT-2.0-0! 627495e0()
screen_display_destroy_diag(_GtkWidget * 0x009800f0, _GtkWidget * 0x00936508) line 8670 + 9 bytes
LIBGOBJECT-2.0-0! 62743995()
LIBGOBJECT-2.0-0! 62757195()
LIBGOBJECT-2.0-0! 62757f3e()
LIBGOBJECT-2.0-0! 627581b6()
LIBGTK-WIN32-2.0-0! 604a56e0()
LIBGTK-WIN32-2.0-0! 604a793c()
LIBGOBJECT-2.0-0! 62743995()
LIBGOBJECT-2.0-0! 62756b92()
LIBGOBJECT-2.0-0! 62757f3e()
LIBGOBJECT-2.0-0! 627581b6()
LIBGTK-WIN32-2.0-0! 604a5650()
LIBGTK-WIN32-2.0-0! 604a6d52()
LIBGTK-WIN32-2.0-0! 6058c212()
LIBGOBJECT-2.0-0! 62743995()
LIBGOBJECT-2.0-0! 62756dc6()
LIBGOBJECT-2.0-0! 62757c9c()
LIBGOBJECT-2.0-0! 627581b6()
LIBGTK-WIN32-2.0-0! 606bde44()
LIBGTK-WIN32-2.0-0! 605893c1()
LIBGTK-WIN32-2.0-0! 6058a6e1()
LIBGDK-WIN32-2.0-0! 6b07114e()
LIBGLIB-2.0-0! 672de877()
LIBGLIB-2.0-0! 672dfd5b()
LIBGLIB-2.0-0! 672dff3a()
LIBGTK-WIN32-2.0-0! 60589c2e()
main(int 0x00000001, char * * 0x00344da0) line 1382

Other information:
Environment:
 Windows gtk & etc from ftp.gnome.org (as linked from Tor Lillqvist's site)
 Windows MSVC6
 Windows XP

The problem was originally encountered when the development Windows Wireshark began to use the latest Windows Gtk/Glib Dll's & etc.
(See http://bugs.wireshark.org/bugzilla/show_bug.cgi?id=2066 for details).

The problem seems to have been introduced in GTK 2.12.2/Glib 2.14.4. 
(The problem still exists with gtk 2.12.3/glib 2.14.4).
The problem does not occur with GTK 2.12.1/Glib 2.14.3

The problem does not seem to show up on Fedora 32-bits using 
GTK 2.12.2/Glib 2.14.4.

The problem can also be demonstrated with a very simple gtk program:

--------------
#include <gtk/gtk.h>

static void destroy_quit( GtkWidget *w, gpointer d ) {
    gtk_main_quit();
}

static void destroy_win( GtkWidget *w, gpointer d) {
	gtk_widget_destroy(w);
}

int main( int   argc, char *argv[] ) {
    GtkWidget *w, *cw, *combo;
    GList *items = NULL;
    
    gtk_init (&argc, &argv);
    
    w = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT (w), "destroy", G_CALLBACK (destroy_quit), NULL);
    gtk_widget_show_all(w);
	
    cw = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT (cw), "destroy", G_CALLBACK (destroy_win) , NULL);
	
    combo = gtk_combo_new();
    items = g_list_append(items, "First Item");
    gtk_combo_set_popdown_strings(GTK_COMBO (combo), items);
    gtk_container_add(GTK_CONTAINER (cw), combo);
    gtk_widget_show_all(cw);
    
    gtk_main ();
    
    return 0;
}
-----------------------------
run program, invoke popdown and then delete (destroy) the window with the GtkCombo widget. The program will crash emitting the following message:

(gtk_combo_test.exe:3764): GLib-GObject-WARNING **: invalid uninstantiatable typ
e `<invalid>' in cast to `GdkWindow'

The crash actually occurs *only* if the popdown has been displayed when calling gtk_widget_destroy on the window containing the GtkCombo widget .