Steps to reproduce the issue

Installing Joomla 3.9 (from 3.8.13) using built-in Joomla Update. Clicked normal blue "Install the Update" button.

Expected result

Notice of successful upgrade from 3.8.13 to 3.9.0

Actual result

Receive error message: "Error: 0 Class 'JToolbarHelper not found'. There is a button to return to the Control Panel. Pushing the button just re-displays the same screen. Joomla menu is visible (System, Users, Menu, etc.). No matter what option I select, this same screen re-displays.

System information (as much as possible)

From Joomla Update log file:

2018-10-30T15:16:44+00:00 INFO 184.182.14.245 update Update started by user Super User (XXX). Old version is 3.8.13.
2018-10-30T15:16:46+00:00 INFO 184.182.14.245 update Downloading update file from https://s3-us-west-2.amazonaws.com/joomla-official-downloads/joomladownloads/joomla3/Joomla_3.9.0-Stable-Update_Package.zip?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIZ6S3Q3YQHG57ZRA%2F20181030%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20181030T151639Z&X-Amz-Expires=60&X-Amz-SignedHeaders=host&X-Amz-Signature=a9ddfa7d6588177fa0393686ff54f0bfda1bcc35c398587e73136e342327f237.
2018-10-30T15:17:02+00:00 INFO 184.182.14.245 update File Joomla_3.9.0-Stable-Update_Package.zip downloaded.
2018-10-30T15:17:03+00:00 INFO 184.182.14.245 update Starting installation of new version.

Additional comments

Website is still running apparently okay. But no access to the backend to maintain.