fix undefined variable

html/index.php

@@ -176,14 +176,17 @@ if (isset($_POST['xd_check'])){
       <nav id="languages"><a href="<?php echo $currentArena; ?>-fr">fr</a>&nbsp;<a href="<?php echo $currentArena; ?>-en">en</a></nav>
       <nav id="menus"><a href="/"<?php if(($currentArena == "") && (!isset($_GET['doc']))) echo ' class="selected"'; ?>><?php echo $lang['HOME']; ?></a>
       <?php
-            foreach($arenas as $arena){
-                if( $arena['id'] == $currentArena){
-                    $class="selected";
-                }else{
-                    $class="";
-                }
-                echo '<a href="'.$arena['url'].'" class="'.$class.'">'.$arena['title'].'</a>';
+        if(!isset($currentArena)){
+            $currentArena="";
+        }
+        foreach($arenas as $arena){
+            if( $arena['id'] == $currentArena){
+                $class="selected";
+            }else{
+                $class="";
             }
+            echo '<a href="'.$arena['url'].'" class="'.$class.'">'.$arena['title'].'</a>';
+        }
       ?></nav>
   </header>
   <section>