fix: prevent warning when printing username

templates/LoginForm.php

@@ -87,7 +87,7 @@ if (null !== $this->data['errorCode']) {
                                     <?php
                                     } ?>
                                     <input type="hidden" id="username" name="username"
-                                           value="<?php echo htmlspecialchars($this->data['username'], ENT_QUOTES); ?>"/>
+                                           value="<?php echo htmlspecialchars($this->data['username'] ?? '', ENT_QUOTES); ?>"/>
                                     <?php
                                 } else {
                                     ?>