fait disparaitre l'erreur php

facto.php

@@ -79,7 +79,7 @@ function header2($where){
 					<div class="forms-w3ls-agilesm text-center mt-lg-0 mt-4">
 						<ul>
 							<li class="login-list-w3ls d-inline border-right pr-3 mr-3">';
-                 if (!$_SESSION['username'])
+                 if (!isset($_SESSION['username']))
                         echo '<a id="modal_trigger" href="#modal" class="text-white">Login | Register</a>';
                       else {
                         echo '<span style="color: #fff; letter-spacing: 2px; font-size: 15px; padding: 0.5em 0; font-family: "Source Sans Pro", sans-serif"><a href="userpage.php" style="color: #fff; letter-spacing: 2px; font-size: 15px; padding: 0.5em 0; font-family: "Source Sans Pro", sans-serif">'.$_SESSION['username'] .'</a> | <a href="logout.php" style="color: #fff; letter-spacing: 2px; font-size: 15px; padding: 0.5em 0; font-family: "Source Sans Pro", sans-serif">Log out</a></span>';