Don't crash when having an update from an unknown identity

ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm

 // Copyright 2015 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.
 
 #import "ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.h"
 
 #include <memory>
 
 #include "base/auto_reset.h"
 #include "base/mac/foundation_util.h"
@@ skipping 946 matching lines @@
   [self stopBrowserStateServiceObservers];
   [_authenticationFlow cancelAndDismiss];
 }
 
 #pragma mark - ChromeIdentityServiceObserver
 
 - (void)onProfileUpdate:(ChromeIdentity*)identity {
   CollectionViewAccountItem* item =
       base::mac::ObjCCastStrict<CollectionViewAccountItem>(
           [_identityMap objectForKey:identity.gaiaID]);
+  if (!item) {
+    // Ignoring unknown identity.
+    return;
+  }
   [self updateAccountItem:item withIdentity:identity];
   [self reconfigureCellsForItems:@[ item ]
          inSectionWithIdentifier:SectionIdentifierSyncAccounts];
 }
 
 - (void)onChromeIdentityServiceWillBeDestroyed {
   _identityServiceObserver.reset();
 }
 
 @end