diff --git a/include/plugin_api.inc.php b/include/plugin_api.inc.php
index 43dca2f4..e37f04b8 100644
--- a/include/plugin_api.inc.php
+++ b/include/plugin_api.inc.php
@@ -526,7 +526,7 @@ class serendipity_plugin_api
         global $serendipity;
 
         // Can be shortcircuited via a $serendipity['prevent_sidebar_plugins_(left|right|event)'] variable!
-        if (!$negate && $serendipity['prevent_sidebar_plugins_' . $filter] == true) {
+        if (!$negate && ($serendipity['prevent_sidebar_plugins_' . $filter] ?? false) == true) {
             return 0;
         }
 
@@ -1144,7 +1144,7 @@ class serendipity_plugin_api
         // skip the execution of any follow-up plugins.
         $plugins = serendipity_plugin_api::get_event_plugins();
 
-        if ($serendipity['core_events'][$event_name]) {
+        if (array_key_exists($event_name, $serendipity['core_events']) && $serendipity['core_events'][$event_name]) {
             foreach($serendipity['core_events'][$event_name] as $apifunc_key => $apifunc) {
                 $apifunc($event_name, $bag, $eventData, $addData);
             }
@@ -1308,9 +1308,9 @@ class serendipity_plugin_api
         $probelang = $path . '/' . $serendipity['charset'] . 'lang_' . $serendipity['lang'] . '.inc.php';
         if (file_exists($probelang)) {
             include $probelang;
+        } else {
+            include $path . '/lang_en.inc.php';
         }
-
-        include $path . '/lang_en.inc.php';
     }
 }
 
@@ -1539,13 +1539,13 @@ class serendipity_plugin
                     break;
 
                 case 'url':
-                    if (!preg_match('§^' . $pattern_url . '$§', $value)) {
+                    if (!preg_match('§^' . $pattern_url . '$§', $value)) {
                         $valid = false;
                     }
                     break;
 
                 case 'mail':
-                    if (!preg_match('§^' . $pattern_mail . '$§', $value)) {
+                    if (!preg_match('§^' . $pattern_mail . '$§', $value)) {
                         $valid = false;
                     }
                     break;
@@ -1797,7 +1797,7 @@ class serendipity_event extends serendipity_plugin
      * @param   array       The entry superarray to get the reference from
      * @return  array       The value of the array for the fieldname (reference)
      */
-    function &getFieldReference($fieldname = 'body', &$eventData)
+    function &getFieldReference($fieldname = 'body', &$eventData = [])
     {
         // Get a reference to a content field (body/extended) of
         // $entries input data. This is a unifying function because