Fix a crash when we try to close a js dialog that wasn't shown.

chrome/browser/app_modal_dialog_gtk.cc

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/app_modal_dialog.h"
 
 #include <gtk/gtk.h>
 
 #include "app/l10n_util.h"
 #include "app/message_box_flags.h"
@@ skipping 13 matching lines @@
 void AppModalDialog::CreateAndShowDialog() {
 }
 
 // static
 void AppModalDialog::OnDialogResponse(GtkDialog* dialog, gint response_id,
                                       AppModalDialog* app_modal_dialog) {
   app_modal_dialog->HandleDialogResponse(dialog, response_id);
 }
 
 void AppModalDialog::ActivateModalDialog() {
+  DCHECK(dialog_);
   gtk_window_present(GTK_WINDOW(dialog_));
 }
 
 void AppModalDialog::CloseModalDialog() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_DELETE_EVENT);
 }
 
 void AppModalDialog::AcceptWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_OK);
 }
 
 void AppModalDialog::CancelWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_CANCEL);
 }