diff --git a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
index f17df18..10fc04a 100644
--- a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
@@ -875,10 +875,12 @@ public function buildOptionsForm(&$form, FormStateInterface $form_state) {
       $this->documentSelfTokens($options[t('Fields')]);
 
       // Default text.
-      $output = '<p>' . $this->t('You must add some additional fields to this display before using this field. These fields may be marked as <em>Exclude from display</em> if you prefer. Note that due to rendering order, you cannot use fields that come after this field; if you need a field not listed here, rearrange your fields.') . '</p>';
+
+      $output = [];
+      $output[][ '#markup'] = '<p>' . $this->t('You must add some additional fields to this display before using this field. These fields may be marked as <em>Exclude from display</em> if you prefer. Note that due to rendering order, you cannot use fields that come after this field; if you need a field not listed here, rearrange your fields.') . '</p>';
       // We have some options, so make a list.
       if (!empty($options)) {
-        $output = '<p>' . $this->t("The following replacement tokens are available for this field. Note that due to rendering order, you cannot use fields that come after this field; if you need a field not listed here, rearrange your fields.") . '</p>';
+        $output[]['#markup'] = '<p>' . $this->t("The following replacement tokens are available for this field. Note that due to rendering order, you cannot use fields that come after this field; if you need a field not listed here, rearrange your fields.") . '</p>';
         foreach (array_keys($options) as $type) {
           if (!empty($options[$type])) {
             $items = array();
@@ -890,7 +892,7 @@ public function buildOptionsForm(&$form, FormStateInterface $form_state) {
               '#items' => $items,
               '#list_type' => $type,
             );
-            $output .= $this->getRenderer()->render($item_list);
+            $output[] = $item_list;
           }
         }
       }
@@ -901,7 +903,7 @@ public function buildOptionsForm(&$form, FormStateInterface $form_state) {
       $form['alter']['help'] = array(
         '#type' => 'details',
         '#title' => $this->t('Replacement patterns'),
-        '#value' => SafeMarkup::set($output),
+        '#value' => $output,
         '#states' => array(
           'visible' => array(
             array(
diff --git a/core/modules/views_ui/src/Tests/FieldUITest.php b/core/modules/views_ui/src/Tests/FieldUITest.php
index 81d2b7e..34d4215 100644
--- a/core/modules/views_ui/src/Tests/FieldUITest.php
+++ b/core/modules/views_ui/src/Tests/FieldUITest.php
@@ -57,6 +57,11 @@ public function testFieldUI() {
     $this->assertEqual((string) $result[0], '{{ age }} == Age');
     $this->assertEqual((string) $result[1], '{{ id }} == ID');
     $this->assertEqual((string) $result[2], '{{ name }} == Name');
+
+    // Ensure the AJAX UI is responding.
+    $ajax_url = 'admin/structure/views/ajax/handler/test_view/default/field/age';
+    $this->drupalGet($ajax_url);
+    $this->assertResponse(200);
   }
 
   /**