IncorrectConfigTargetForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
class IncorrectConfigTargetForm extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'form_test.object',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_incorrect_config_target_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['missing_key'] = [
            '#type' => 'textfield',
            '#title' => t('Missing key'),
            '#config_target' => 'form_test.object:does_not_exist',
        ];
        return parent::buildForm($form, $form_state);
    }

}