errors and broken cron if $dom_node in TocBuilder is null

Problem/Motivation

Code errors at line 80 of src/TocBuilder.php if $dom_node is null.is

Steps to reproduce

First seen when clicking on 'Place block' buttons on block layout form and getting no response.

Create a node where a Table of Contents is after the first heading tag. (We place a summary of the node before the TOC.) Use TOC Filter module. The first heading tag is a DOMNode with an id added by TOC Filter, but it has no parent.

Proposed resolution

Change
$dom_node->parentNode->replaceChild($fragment_node, $dom_node);
to

if($dom_node->parentNode) {
  $dom_node->parentNode->replaceChild($fragment_node, $dom_node);
} 

NB checking $dom_node is not null does not fix the issue, which is with a valid $dom_node with no parent. PHP 7.4.3

Remaining tasks

Make patch or PR or just update dev version.