Deprecated function: html_entity_decode(): Passing null to parameter #1 ($string) of type string is deprecated in metatag_preprocess_html() (line 467 of metatag.module).

Thank you for reporting this bug. Can you please test to see if the current dev version resolves the problem. Thank you.

Created MR!79 for the issue mentioned, please review.
Thank you.

Can you please explain how this happens? Did you set the page title tag to only include a token that ends up being blank when it's rendered? Have you tried looking at the contents of the $attachment variable inside the if() statement in this part to see what values are present?

    foreach ($metatag_attachments['#attached']['html_head'] as $attachment) {
      if (!empty($attachment[1]) && $attachment[1] == 'title') {
        // Empty head_title to avoid the site name and slogan to be appended to
        // the meta title.
        $variables['head_title'] = [];
        $variables['head_title']['title'] = html_entity_decode($attachment[0]['#attributes']['content'], ENT_QUOTES);

        // No need to do anything else after this.
        break;
      }
    }

Need some feedback on what caused this problem.

Now with the empty page or user profil and any page $attachment[0]['#attributes']['content'] was null

The proposal of Ranjit1032002 was great choice for me

Ok, that's fair.

Committed, with a minor tweak.

Automatically closed - issue fixed for 2 weeks with no activity.