Added element_validate() function to remove keys from the form state that are...

Added element_validate() function to remove keys from the form state that are not part of the markdown config.

Closes #3470570

src/Plugin/Filter/FilterMarkdown.php

@@ -419,9 +419,29 @@ class FilterMarkdown extends FilterBase implements ContainerFactoryPluginInterfa
       $parserElement['id']['#description'] = $this->moreInfo($parserElement['id']['#description'], $url);
     }
 
+    // The filter module expects that every form element is a configuration key.
+    // Add a validation function to remove values from the form state that
+    // should not be saved as configuration.
+    $element['#element_validate'][] = [$this, 'subformValidate'];
+
     return $element;
   }
 
+  /**
+   * Removes certain keys from the markdown settings.
+   *
+   * @param array $element
+   *   The element being validated.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current form state.
+   */
+  public function subformValidate(array $element, FormStateInterface $form_state) {
+    $markdown_settings = &$form_state->getValue(['filters', 'markdown', 'settings']);
+
+    unset($markdown_settings['vertical_tabs']);
+    unset($markdown_settings['error']);
+  }
+
   /**
    * The AJAX callback used to return the parser ajax wrapper.
    */