ValidReferenceConstraintValidator causes a fatal error when $item->entity isn't present

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I got this error too when I tried to set up a computed field programmatically (no use of the Compute Field → contrib module) in one of my content types.

Here is a simplified way to reproduce the fatal error in a similar context.
You'll just have to create a very simple custom computed field for the Basic page content type.
You can use any of your custom modules or create one for testing purpose.

1- Custom computed field declaration in the .module file

function my_custom_module_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
  $fields = [];
  if ($entity_type->id() == 'node') {
    if ($bundle == 'page') {
      $fields['resources'] = BaseFieldDefinition::create('link')
        ->setLabel(t('Resources'))
        ->setComputed(TRUE)
        ->setClass('\Drupal\my_custom_module\ResourcesLinksComputed')
        ->setDisplayConfigurable('view', TRUE)
        ->setReadOnly(true);
    }
  }
  return $fields;
}

2- Creation of the custom class to handle the field value calculation
In the file web/modules/custom/my_custom_module/src/ResourcesLinksComputed.php, add the following lines :

class ResourcesLinksComputed extends EntityReferenceFieldItemList
{
  use ComputedItemListTrait;

  protected function computeValue() {
    $entity = $this->getEntity();

    if ($entity->isNew()) {
      return;
    }

      $link = array(
        'uri' => 'https://www.drupal.org',
        'title' => "Drupal web site",
        'options' => array(),
      );

    $this->list[0] = $this->createItem(0, $link);
  }
}

3- Testing your new computed field
Go to Manage display admin page of the Basic page content type (.../admin/structure/types/manage/page/display).
You will see the new field "Resources links" in the "Disabled" fields list.
Drag and drop this field right after the enabled field(s) and save settings.
Visit any of your basic pages and you will see the Drupal web site → link a the bottom of the page.

4- Visualizing the fatal error
Still on your basic page, click on the Edit tab.
No need to modify any content, you just have save the page and ... Pow !
White screen with the same error message as mentionned by @lmncn :

Error: Call to a member function isNew() on null in Drupal\Core\Entity\Plugin\Validation\Constraint\ValidReferenceConstraintValidator->validate() (line 74 of core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php). 

Now, if you pass Null instead of $link in the createItem() fonction (line 27 of ResourcesLinksComputed.php), the bug disappears.
And, as proposed by @lmncn, replacing

if (!$item->entity->isNew()) {

by

if (!$item->getEntity()->isNew()) {

in the core web/core/lib/Drupal/Core/Entity/Plugin/Validation/Constraint/ValidReferenceConstraintValidator.php file solves the problem as well.

I had this same issue but now realize it was due to a mistake I had made, and it's the same mistake @jabberwooki made in #12. The computed field class for the link should extend `\Drupal\Core\Field\FieldItemList`, not `\Drupal\Core\Field\EntityReferenceFieldItemList`. It was due to this mistake that Drupal was calling ValidReferenceConstraintValidator on this link field which caused the crash in the issue summary.

After extending from the correct class I removed the patch in #2 from my site and I'm still able to save the node without the crash.