MainNavBar: harden Registry access

templates/MainNavBar.php

@@ -33,7 +33,7 @@ class MainNavBar extends NavBar
 					<span class="navbar-toggler-icon"></span>
 				</button>';
 
-		if (Registry::get('user')->isLoggedIn())
+		if (Registry::has('user') && Registry::get('user')->isLoggedIn())
 		{
 			echo '
 				<div class="collapse navbar-collapse justify-content-end" id="', $this->innerMenuId, '">