ViewsTestDataErrorForm.php

<?php

namespace Drupal\views_test_data\Form;

use Drupal\Core\Form\FormInterface;
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements a test form that has a validation error.
 */
class ViewsTestDataErrorForm implements FormInterface {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'views_test_data_error_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['text'] = [
            '#type' => 'textfield',
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $form_state->setErrorByName('text', t('Form validation error'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}