Fix #15540 - Uncaught TypeError: htmlspecialchars() in NavigationTree

Signed-off-by: William Desportes <williamdes@wdes.fr>

libraries/classes/Navigation/NavigationTree.php

@@ -764,7 +764,7 @@ class NavigationTree
                 }
 
                 $groups[$key] = new Node(
-                    htmlspecialchars($key),
+                    htmlspecialchars((string) $key),
                     Node::CONTAINER,
                     true
                 );