From 36dcf5e6f56b34097677cf090b27bdac347a665c Mon Sep 17 00:00:00 2001
From: "M. Vondano" <m-vo@users.noreply.github.com>
Date: Thu, 25 Feb 2021 19:09:38 +0100
Subject: [PATCH] fix accessing undefined variable

---
 core-bundle/src/Resources/contao/classes/Backend.php | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/core-bundle/src/Resources/contao/classes/Backend.php b/core-bundle/src/Resources/contao/classes/Backend.php
index dffcda9b1b9..ef5834b5b93 100644
--- a/core-bundle/src/Resources/contao/classes/Backend.php
+++ b/core-bundle/src/Resources/contao/classes/Backend.php
@@ -894,7 +894,7 @@ public static function addPagesBreadcrumb($strKey='tl_page_node')
 		$arrLinks = array_reverse($arrLinks);
 
 		// Insert breadcrumb menu
-		$GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] .= '
+		$GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] = ($GLOBALS['TL_DCA']['tl_page']['list']['sorting']['breadcrumb'] ?? '') . '
 
 <nav aria-label="' . $GLOBALS['TL_LANG']['MSC']['breadcrumbMenu'] . '">
   <ul id="tl_breadcrumb">
@@ -1068,7 +1068,7 @@ public static function addFilesBreadcrumb($strKey='tl_files_node')
 		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['root'] = array($strNode);
 
 		// Insert breadcrumb menu
-		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] .= '
+		$GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] = ($GLOBALS['TL_DCA']['tl_files']['list']['sorting']['breadcrumb'] ?? '') . '
 
 <nav aria-label="' . $GLOBALS['TL_LANG']['MSC']['breadcrumbMenu'] . '">
   <ul id="tl_breadcrumb">