Fix reading property of null object in chrome extension's updateEmbedElement function

[erm1116]

We have faced the problem that PDF Viewer extension does not work correctly in the following cases

<div>
  <embed src="..." />
</div>

The error output to the console is as follows

contentscript.js:136 Uncaught TypeError: Cannot read properties of null (reading 'before')
    at updateEmbedElement (contentscript.js:136:17)
    at updateViewerFrame (contentscript.js:99:11)
    at watchObjectOrEmbed (contentscript.js:108:3)
    at HTMLDocument.onAnimationStart (contentscript.js:33:5)

Checking the previous changes,
it looks like the change is from parentNode.insertBefore(elem, nextSibling) to nextSibling.before(elem).
In the case of insertBefore, if nextSibling is null, it is added as the end of the node's child, so it behaves differently.

Therefore, if nextSibling does not exist, adding the node to the end of the child by parentNode.append.

Reference:
Node.insertBefore()
Element.before()

[Rob--W]

Thanks for this patch.

This might be the cause of #16594.

[Rob--W]

I would have used the insertBefore instead of before/append because that method did exactly what it was supposed to. But this works too, so r+.

[erm1116]

Yes, I think insertBefore would work.
But since the Lint rule was set, I use before/append to solve this problem.

[timvandermeij]

Thanks!