PHP8 Error : Undefined array key "richtext" fix

Author: lazar2038

core/functions/actions/mutate_content.php

@@ -883,7 +883,7 @@ function renderFormElement(
                     global $_lang;
                     global $ResourceManagerLoaded;
                     global $content, $which_editor;
-                    if (!$ResourceManagerLoaded && !(($content['richtext'] == 1 || $modx->getManagerApi()->action == 4) && $modx->getConfig('use_editor') && $which_editor == 3)) {
+                    if (!$ResourceManagerLoaded && !(((isset($content['richtext']) && $content['richtext'] == 1) || $modx->getManagerApi()->action == 4) && $modx->getConfig('use_editor') && $which_editor == 3)) {
                         $ResourceManagerLoaded = true;
                     }
                     $field_html .= '<input type="text" id="tv' . $field_id . '" name="tv' . $field_id . '"  value="' . $field_value . '" ' . $field_style . ' onchange="documentDirty=true;" /><input type="button" value="' . $_lang['insert'] . '" onclick="BrowseFileServer(\'tv' . $field_id . '\')" />';