DOMLex error when changing theme appearance

We are having an issue when trying to edit our theme through the Omeka admin. After changing any value and selecting “Save Changes”, a list of errors appear that all state:

Notice: Undefined property: DOMText::$data in /var/www/application/libraries/htmlpurifier/HTMLPurifier/Lexer/DOMLex.php on line 136

It repeats this error a number of times on the Omeka admin Appearance page. It also clears the entire Appearance form. No changes to the theme are applied. We are using the default Thanks, Roy theme.