FieldStorageAddForm::validateAddNew

protected FieldStorageAddForm::validateAddNew(array $form, FormStateInterface $form_state)

protected function validateAddNew(array $form, FormStateInterface $form_state) {
  // Validate if any information was provided in the 'add new field' case.
  if ($form_state->getValue('new_storage_type')) {
    // Missing label.
    if (!$form_state->getValue('label')) {
      $form_state->setErrorByName('label', $this->t('Add new field: you need to provide a label.'));
    }

    // Missing field name.
    if (!$form_state->getValue('field_name')) {
      $form_state->setErrorByName('field_name', $this->t('Add new field: you need to provide a machine name for the field.'));
    }
    // Field name validation.
    else {
      $field_name = $form_state->getValue('field_name');

      // Add the field prefix.
      $field_name = $this->configFactory->get('field_ui.settings')->get('field_prefix') . $field_name;
      $form_state->setValueForElement($form['new_storage_wrapper']['field_name'], $field_name);
    }
  }
}