When adding fields to a table that exists, if NOT NULL is checked, there is an error.
If your table already contains rows and you add a column with a NOT NULL constraint, you also need to provide a DEFAULT value (that is not NULL). Otherwise, existng rows violate the NOT NULL constraint and thus you get an Error.
This is not a bug, it is a mistake of the user.
Please next time you post a bug Report: copy the exact error messge! And read it. Don't just say "there is an error".