BUGFIX: Make BackendUtility usable in frontend

BackendUtility used LanguageService which only works
in the backend.
Extend BackendUtility and use TSFE instead.

Classes/Domain/Index/TcaIndexer/RelationResolver.php

@@ -20,6 +20,7 @@ namespace Codappix\SearchCore\Domain\Index\TcaIndexer;
  * 02110-1301, USA.
  */
 
+use Codappix\SearchCore\Utility\FrontendUtility;
 use TYPO3\CMS\Backend\Utility\BackendUtility;
 use TYPO3\CMS\Core\SingletonInterface as Singleton;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
@@ -39,13 +40,15 @@ class RelationResolver implements Singleton
             if ($column === 'pid') {
                 continue;
             }
-            $record[$column] = BackendUtility::getProcessedValueExtra(
+
-                $service->getTableName(),
+            $record[$column] = GeneralUtility::makeInstance($this->getUtilityForMode())
-                $column,
+                ::getProcessedValueExtra(
-                $record[$column],
+                    $service->getTableName(),
-                0,
+                    $column,
-                $record['uid']
+                    $record[$column],
-            );
+                    0,
+                    $record['uid']
+                );
 
             try {
                 $config = $service->getColumnConfig($column);
@@ -93,4 +96,13 @@ class RelationResolver implements Singleton
     {
         return array_map('trim', explode(',', $value));
     }
+
+    protected function getUtilityForMode(): string
+    {
+        if (TYPO3_MODE === 'BE') {
+            return BackendUtility::class;
+        }
+
+        return FrontendUtility::class;
+    }
 }

Classes/Utility/FrontendUtility.php

@@ -0,0 +1,39 @@
+<?php
+namespace Codappix\SearchCore\Utility;
+
+/*
+ * Copyright (C) 2018 Justus Moroni <developer@leonmrni.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+use TYPO3\CMS\Backend\Utility\BackendUtility;
+use TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController;
+
+/**
+ * Overwrite BackendUtility to use in frontend.
+ * LanguageService was only usable in backend.
+ */
+class FrontendUtility extends BackendUtility
+{
+    /**
+     * @return TypoScriptFrontendController
+     */
+    protected static function getLanguageService()
+    {
+        return $GLOBALS['TSFE'];
+    }
+}