[regression] JavaScript TypeError when a contextual link placeholder is moved or emptied

Problem/Motivation

It looks like #3203920: Replace Contextual Links BackboneJS usage with VanillaJS equivalent caused a regression in specific scenarios.

After updating from D10.4 to 11.2, I see the following JS error:

Uncaught TypeError: Cannot set properties of undefined (setting 'textContent')
    at ContextualModelView.render (contextual.js?v=11.2.2:356:29)
    at new ContextualModelView (contextual.js?v=11.2.2:331:14)
    at initContextual (contextual.js?v=11.2.2:113:33)
    at contextual.js?v=11.2.2:167:13

This seems to happen when some HTML that contains contextual links are being relocated in JS.
On my project, it happens with mmenu JS external lib which builds the mobile menu.
It clones a ul (which contains blocks inside) and moves it at the bottom of the HTML, right before the closing body tag.
I did manage to reproduce with olivero, see below.

Steps to reproduce

- Install standard profile on 11.x branch
- Add the following behavior, anywhere

Drupal.behaviors.contextual_bug = {
  attach(context) {
    once('contextual_bug', '#block-olivero-views-block-who-s-online-who-s-online-block .block__content', context).forEach((element) => {
      // Move the who's online block content at the bottom, before body closing tag, replicating mmenu.
      const clone = element.cloneNode(true);
      document.body.appendChild(clone);
      element.remove();
    });
  }
}

(I place it in core/modules/contextual/js/contextual.js:537 at the bottom, but in the IIFE)
- Login as uid1, add the "Who's online" block in the content region
Note: why that block? because it seems there is a relation with big_pipe replacement and the block contains a .contextual-region element within it's content, both are required to reproduce the bug.
- Clear cache, hit the homepage (still logged in)
>>> The JS error is thrown

Proposed resolution

The code at core/modules/contextual/js/contextual.js:167 is attempting to get data-contextual-id elements that are not guaranteed to be there, in case these were relocated.

window.setTimeout(() => {
  initContextual(
    $context
      .find(`[data-contextual-id="${contextualID.id}"]:empty`)
      .eq(0),
    html,
  );
});

This line hasn't changed in #3203920 but the same happens before that.
The main difference is that all jQuery.each and selectors were silently skipping in initContextual and ContextualModelView before. But now, ContextualModelView::render was changed and hard fails on trigger.textContent assignment.
I suggest to add some defensive checks in a few places.

Remaining tasks

N/A

User interface changes

N/A

Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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