The "node" entity cannot have a URI as it does not have an ID in Drupal\Core\Entity\EntityBase->toUrl() (line 161 of core/lib/Drupal/Core/Entity/EntityBase.php).

Something strange has happened. I created the MR but when I clicked on the link after the push I had no access. I passed it to my partner paucala and he was able to access it.

This is a bit strange, as this code is more-or-less taken from core Content Translation module. So, you'd think you would also see this error when Content Translation is in use.

However, I can look into trying to reproduce this...

I looked into this issue, and found that the code in Content Translation module would trigger the same exception. And so, apparently, Mercury Editor decided to simply disable the hook_page_attachments() implementation for Content Translation module, rather than properly fixing the issue. Therefore, you won't have any hreflang tags provided by Content Translation module if you enable Mercury Editor. So, that seems like bugs that need to be fixed in both Content Translation module and Mercury Editor.

In any case, we can resolve the issue in Hreflang module, at least...

mfb → closed merge request !4

Actually, looks like Mercury Editor re-implemented the Content Translation page attachments, so you do get hreflang links after all. So, just bug in core Content Translation module I guess.