What is the cause of the PHP Fatal error: Uncaught Error: Class 'WP_Widget_Hook' not found in the file located in /home/terrahom/public_html/wp-includes/class-wp-widget.php at line 173?

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This error occurs when the WordPress instance cannot find the class 'WPWidgetHook' which is used in the widget API. It is often caused by outdated or missing files in the WordPress installation, usually due to a failed update or migration process. It can also be caused by a conflict with a plugin or theme that is not compatible with the current version of WordPress. To resolve this issue, updating the WordPress installation or disabling incompatible plugins and themes is often necessary.