Fatal Error Caused by Enabling user activation

Hi
When in User Registration I enable user activation and submit the form, causes this error and makes the website down:

PHP Notice: Undefined property: DOMDocument::$documentElement in /home/*********/public_html/wp-content/plugins/wpforms/vendor/tijsverkoyen/css-to-inline-styles/src/CssToInlineStyles.php on line 132
PHP Fatal error: Uncaught TypeError: Argument 1 passed to DOMNode::removeChild() must be an instance of DOMNode, null given in /home/*********/public_html/wp-content/plugins/wpforms/vendor/tijsverkoyen/css-to-inline-styles/src/CssToInlineStyles.php:137
Stack trace:

0 /home/*********/public_html/wp-content/plugins/wpforms/vendor/tijsverkoyen/css-to-inline-styles/src/CssToInlineStyles.php(137): DOMNode->removeChild(NULL)

1 /home/*********/public_html/wp-content/plugins/wpforms/vendor/tijsverkoyen/css-to-inline-styles/src/CssToInlineStyles.php(50): TijsVerkoyen\CssToInlineStyles\CssToInlineStyles->getHtmlFromDocument(Object(DOMDocument))

2 /home/*********/public_html/wp-content/plugins/wpforms/src/Emails/Styler.php(115): TijsVerkoyen\CssToInlineStyles\CssToInlineStyles->convert(‘\n

3 /home/*********/public_html/wp-content/plugins/wpforms/src/Emails/Styler.php(133): WPForms\Emails\Styler->process_markup()

4 /home/*********/public_html/wp-content/plugins/wpforms/src/Emails/Templates/General.php(415): WP in /home/*********/public_html/wp-content/plugins/wpforms/vendor/tijsverkoyen/css-to-inline-styles/src/CssToInlineStyles.php on line 137