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

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

Index: ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm
diff --git a/ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm b/ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm
index 0632a09b8558ae51d52bbdb29207a196537b345e..eb123e3879c06e36e7e9dfa3b4d7816770416163 100644
--- a/ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm
+++ b/ios/chrome/browser/ui/settings/sync_settings_collection_view_controller.mm
@@ -964,6 +964,10 @@ typedef NS_ENUM(NSInteger, ItemType) {
   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];