diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 08160b7..63ba623 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2696,9 +2696,6 @@ function system_preprocess_block(&$variables) {
  */
 function template_preprocess_system_plugin_ui_form(&$variables) {
   drupal_add_css(drupal_get_path('module', 'system') . '/system.plugin.ui.css');
-  $variables['left'] = drupal_render($variables['form']['left']);
-  $variables['right'] = drupal_render($variables['form']['right']);
-  $variables['form_submit'] = drupal_render_children($variables['form']);
 }
 
 /**
diff --git a/core/modules/system/templates/system-plugin-ui-form.html.twig b/core/modules/system/templates/system-plugin-ui-form.html.twig
index 87c03fe..f5b473c 100644
--- a/core/modules/system/templates/system-plugin-ui-form.html.twig
+++ b/core/modules/system/templates/system-plugin-ui-form.html.twig
@@ -4,9 +4,9 @@
  * Default theme implementation to configure blocks.
  *
  * Available variables:
- * - left: Form elements that appear in the left column.
- * - right: Form elements that appear in the right column.
- * - form_submit: Form submit button.
+ * - form: The form.
+ * - form.left: Form elements that appear in the left column.
+ * - form.right: Form elements that appear in the right column.
  *
  * @see template_preprocess_block_library_form()
  * @see theme_block_library_form()
@@ -17,15 +17,15 @@
 <div id="block-library" class="container">
   <div class="left-col">
     <div class="inside">
-      {{ left }}
+      {{ form.left }}
     </div>
   </div>
   <div class="right-col">
     <div class="inside">
-      {{ right }}
+      {{ form.right }}
     </div>
   </div>
   {% if form_submit -%}
-    <div class="bottom-bar">{{ form_submit }}</div>
+    <div class="bottom-bar">{{ form }}</div>
   {%- endif %}
 </div>