The website encountered an unexpected error

Based on →

Description:
\Drupal\Core\Render\Element\RenderElement and \Drupal\Core\Render\Element\FormElement are deprecated and will be removed in Drupal 12. Instead, use \Drupal\Core\Render\Element\RenderElementBase and \Drupal\Core\Render\Element\FormElementBase respectively. This includes all the static methods on both classes.

So when I replaced
use Drupal\Core\Render\Element\FormElementBase;
with
use Drupal\Core\Render\Element\FormElement;

and
class UltimateTable extends FormElementBase implements ContainerFactoryPluginInterface {
with
class UltimateTable extends FormElement implements ContainerFactoryPluginInterface {
I was able at least enable module so no rollback is needed.

However there are 2 options now. Remove D9 compatibility ,or create additional D9 version with method.

The Drupal\Core\Render\Element\FormElementBase class is not available in your current Drupal version. However, the alternative class, Drupal\Core\Render\Element\FormElement, is deprecated in Drupal 10.3 and is scheduled for removal in Drupal 12. To address this, we will continue using the deprecated FormElement class for now, with plans for a Drupal 12-compatible release in the future.

In the meantime, here is a patch to resolve the issue until the next release containing the fix.

I rewrite the path in #3 to also fix Undefined array key "default_formatter" warning.