TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in count() (line 79 of modules/contrib/domain/domain/src/DomainElementManager.php).

Submitted a merge request. Ready for review.

The solution is somewhat doubling the condition check and could be more performant by using nested checks:

instead of

// @@ -76,7 +76,8 @@ class DomainElementManager implements DomainElementManagerInterface {

if ((isset($form[$field_name]['widget']['#options']) &&
    count($form[$field_name]['widget']['#options']) === 0)
    ||
    (isset($form[$field_name]['widget']['#options']) &&
    count($form[$field_name]['widget']['#options']) === 1 &&
    isset($form[$field_name]['widget']['#options']['_none']))
) {
  $empty = TRUE;

maybe like:

// @@ -76,7 +76,8 @@ class DomainElementManager implements DomainElementManagerInterface {

if ((isset($form[$field_name]['widget']['#options']) 
    if (count($form[$field_name]['widget']['#options']) === 0)
	||
        (count($form[$field_name]['widget']['#options']) === 1 &&
        isset($form[$field_name]['widget']['#options']['_none']))
    )
) {
  $empty = TRUE;

I made this condition like this

if ((isset($form[$field_name]['widget']['#options']) &&
         count($form[$field_name]['widget']['#options']) === 0)
        ||
        (isset($form[$field_name]['widget']['#options']) &&
         count($form[$field_name]['widget']['#options']) === 1 &&
         isset($form[$field_name]['widget']['#options']['_none']))
    ) {
      $empty = TRUE;
    } 

While I can confirm the code in #5 does work on D10.3, it is simply a repetition of what's in the merge request mentioned in #3. I agree with dqd in #4 that the change could be rewritten, but the code in that comment isn't quite right and would be a breaking change, so I've tweaked it in the patch attached here, which does work for me.

Here's my +1 comment - We are experiencing this issue and I can confirm that the patch in #6 resolves it.

I also found #6 https://www.drupal.org/project/domain/issues/3470873#comment-15844907 🐛 TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in count() (line 79 of modules/contrib/domain/domain/src/DomainElementManager.php). Active to work, in that I don't now see that fatal error, the title of this issue "TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in count() (line 79 of modules/contrib/domain/domain/src/DomainElementManager.php)". I needed to clear cache.

Thank you!

However, I get a warning on the node add/edit page: "Warning: Undefined array key "#options" in Drupal\domain\DomainElementManager->setFormOptions() (line 79 of modules/contrib/domain/domain/src/DomainElementManager.php)."

But I can get by. Hope for a fix for the warning (or even to help provide one), when I can.

I managed to reproduce this error when using a "Autocomplete" or "Autocomplete (Tags style)" for the field "Domain Access" in the manage form display configuration for the content type.

Clearly the code does not contemplate the possibility of using those field widgets for this field, so the patches do not fix the root cause of the issue.