getDomainId() on null in domain_access_node_create_access() domain/domain_access/domain_access.module:419

I have exactly the same error message when trying to access a add content page in the admin with a user having specific role (for admin it works well).

Here is a patch. It looks like the is_null logic is_numeric are the opposite that needed.
I also take care of the non object that is mentioned in issue https://www.drupal.org/project/domain/issues/2936697 →

oups here is the applicable patch for pervious comment.

Different patch as MR, because I think the base check can be simplified.

In addition, the is_numeric check should not be necessary according to the return type of getDomainId(), so either the check is obsolete, or the function doc is incomplete.
As I don't have the correct answer, have flagged it as a todo.

The MR fixes the issue for me.