[rAc, Mac] Fix a NULL pointer dereference in Mac Autofill dialog.

chrome/browser/ui/cocoa/autofill/autofill_dialog_cocoa.mm

 // Copyright (c) 2013 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/ui/cocoa/autofill/autofill_dialog_cocoa.h"
 
 #include "base/mac/bundle_locations.h"
 #include "base/mac/scoped_nsobject.h"
 #include "chrome/browser/ui/autofill/autofill_dialog_controller.h"
 #include "chrome/browser/ui/chrome_style.h"
@@ skipping 48 matching lines @@
       new ConstrainedWindowMac(this, controller_->web_contents(), sheet));
   constrained_window_->SetPreventCloseOnLoadStart(true);
 }
 
 void AutofillDialogCocoa::Hide() {
   [sheet_controller_ hide];
 }
 
 // Closes the sheet and ends the modal loop. Triggers cleanup sequence.
 void AutofillDialogCocoa::PerformClose() {
-  constrained_window_->CloseWebContentsModalDialog();
+  if (constrained_window_)
+    constrained_window_->CloseWebContentsModalDialog();
 }
 
 void AutofillDialogCocoa::UpdateAccountChooser() {
   [sheet_controller_ updateAccountChooser];
 }
 
 void AutofillDialogCocoa::UpdateButtonStrip() {
 }
 
 void AutofillDialogCocoa::UpdateDetailArea() {
@@ skipping 297 matching lines @@
   [[signInContainer_ view] setHidden:YES];
   [[mainContainer_ view] setHidden:NO];
   [self performLayout];
 }
 
 - (void)modelChanged {
   [mainContainer_ modelChanged];
 }
 
 @end