The sys.fulltext_stoplists doc says the column
principal_id is the ID of the database principal that owns the stoplist. You can specify it as one of the Arguments of the
CREATE FULLTEXT STOPLIST command:
Specifies the name of a database principal to
own of the stoplist. owner_name must either be the name of a principal
of which the current user is a member, or the current user must have
IMPERSONATE permission on owner_name. If not specified, ownership is
given to the current user.
It appears the owner of the object no longer exists. You can assign a new owner using the ALTER AUTHORIZATION like this:
ALTER AUTHORIZATION ON FULLTEXT STOPLIST::CustomStopword TO SomeExistingUser;
After that, run the DBCC check again to see if everything is ok.