I tried to clean the build and rebuild the project a lot of times with the same result. Then, I was tired and I closed Xcode.
Basically, if I close Xcode and reopen the project, the project seems find and Xcode can compile it without showing errors and also I can see the ToDo properties.
I hope I don't have to do the same every time I change the Core Data.
Thank you everybody for the replies.
erossini
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