I found I way to recuperate it without upgrading to 4.13. Maybe it will be useful for someone else.
I moved the backup file inside the content folder and I deleted the saved and config folder. When you open your project, it open the default level, but you can open your backup file the same way you will open a different level in your project. After that, I just saved the level with another name and I deleted the backup file from my content folder.