diff -u -r library/HTMLPurifier/Lexer.php library/HTMLPurifier/Lexer.php
--- library/HTMLPurifier/Lexer.php	2020-06-28 17:56:53.000000000 -0700
+++ library/HTMLPurifier/Lexer.php	2022-10-19 10:27:20.901059891 -0700
@@ -306,7 +306,7 @@
     {
         // normalize newlines to \n
         if ($config->get('Core.NormalizeNewlines')) {
-            $html = str_replace("\r\n", "\n", $html);
+            $html = str_replace("\r\n", "\n", $html ?? '');
             $html = str_replace("\r", "\n", $html);
         }