FieldStorageConfigEditForm::validateForm

public FieldStorageConfigEditForm::validateForm(array &$form, FormStateInterface $form_state)

public function validateForm(array &$form, FormStateInterface $form_state) {
  parent::validateForm($form, $form_state);

  $field_storage_definitions = \Drupal::service('entity_field.manager')->getFieldStorageDefinitions($this->entity->getTargetEntityTypeId());

  // Validate field cardinality.
  if ($form_state->getValue('cardinality') === 'number' && !$form_state->getValue('cardinality_number')) {
    $form_state->setErrorByName('cardinality_number', $this->t('Number of values is required.'));
  }
  // If a specific cardinality is used, validate that there are no entities
  // with a higher delta.
  elseif (!$this->entity->isNew() && isset($field_storage_definitions[$this->entity->getName()]) && $form_state->getValue('cardinality') != FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED) {

    // Get a count of entities that have a value in a delta higher than the
    // one selected. Deltas start with 0, so the selected value does not
    // need to be incremented.
    $entities_with_higher_delta = \Drupal::entityQuery($this->entity->getTargetEntityTypeId())
      ->condition($this->entity->getName() . '.%delta', $form_state->getValue('cardinality'))
      ->count()
      ->execute();
    if ($entities_with_higher_delta) {
      $form_state->setErrorByName('cardinality_number', $this->formatPlural($entities_with_higher_delta, 'There is @count entity with @delta or more values in this field.', 'There are @count entities with @delta or more values in this field.', ['@delta' => $form_state->getValue('cardinality') + 1]));
    }
  }
}