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.