What you must remember at all times is that a project can be saved only if you’re creating it while being logged in to your account. There is no way to save or restore a project that has been made without an account.
The project is being saved automatically. Just give it a couple of seconds before closing the project to send all the changes to servers.
Note: Please make sure your device is connected to the Internet every time you finish working with the project. If you work on the project offline, your changes will only be saved after you connect to the Internet. If you open the project from another device with an active internet connection before you save changes – those will be overwritten.