Issue #1810808 by dawehner, fastangel, damiankloip: Fixed...

Issue #1810808 by dawehner, fastangel, damiankloip: Fixed HandlerBase::buildGroupByForm is still using the display on the viewExecutable.

core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php

@@ -100,7 +100,6 @@ public function init(ViewExecutable $view, DisplayPluginBase $display, array &$o
       $options['field'] = $this->actualField;
     }
 
-    $types = ViewExecutable::viewsHandlerTypes();
     $plural = $this->definition['plugin_type'];
     if (isset($types[$plural]['plural'])) {
       $plural = $types[$plural]['plural'];
@@ -322,9 +321,6 @@ public function buildGroupByForm(&$form, &$form_state) {
     $type = $form_state['type'];
     $id = $form_state['id'];
 
-    $form['#title'] = check_plain($view->display[$display_id]['display_title']) . ': ';
-    $form['#title'] .= t('Configure aggregation settings for @type %item', array('@type' => $types[$type]['lstitle'], '%item' => $this->adminLabel()));
-
     $form['#section'] = $display_id . '-' . $type . '-' . $id;
 
     $view->initQuery();

core/modules/views/views_ui/admin.inc

@@ -1668,7 +1668,7 @@ function views_ui_config_item_group_form($type, &$form_state) {
       $handler->init($executable, $executable->display_handler, $item);
       $types = ViewExecutable::viewsHandlerTypes();
 
-      $form['#title'] = t('Configure group settings for @type %item', array('@type' => $types[$type]['lstitle'], '%item' => $handler->adminLabel()));
+      $form['#title'] = t('Configure aggregation settings for @type %item', array('@type' => $types[$type]['lstitle'], '%item' => $handler->adminLabel()));
 
       $handler->buildGroupByForm($form['options'], $form_state);
       $form_state['handler'] = &$handler;