fix logout bug

plugin.php

@@ -104,7 +104,9 @@ function ldapauth_is_valid_user( $value ) {
 				$yourls_user_passwords[$username]='phpass:ThereIsNoPasswordButHey,WhoCares?';
 			}
 			yourls_set_user( $_SESSION['LDAPAUTH_AUTH_USER'] );
-			return true;
+            //return true; //modified 12.04.2023 by vmario. function must return null to match shunt_is_valid_user in functions-auth.php
+            return null;
+
 		} else {
 			ldapauth_debug($username . ' is not admin user.');
 			return $value;