How to reproduce it again :

Create an empty website
In the CMS create a new page
the assistant is opened, enter the name "index" and save it without going to other steps
the page is well created,
select the page and apply the index template,
the template is well applyied
click on rename the page
> an error occurs :

Line: 243
Erreur : Impossible d’obtenir la propriété « long-title » d’une référence null ou non définie

see screenshots