FieldStorageAddForm::validateForm

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

public function validateForm(array &$form, FormStateInterface $form_state) {
  // Missing field type.
  if (!$form_state->getValue('new_storage_type') && !$form_state->getValue('existing_storage_name')) {
    $form_state->setErrorByName('new_storage_type', $this->t('You need to select a field type or an existing field.'));
  }
  // Both field type and existing field option selected. This is prevented in
  // the UI with JavaScript but we also need a proper server-side validation.
  elseif ($form_state->getValue('new_storage_type') && $form_state->getValue('existing_storage_name')) {
    $form_state->setErrorByName('new_storage_type', $this->t('Adding a new field and re-using an existing field at the same time is not allowed.'));
    return;
  }

  $this->validateAddNew($form, $form_state);
  $this->validateAddExisting($form, $form_state);
}