fix flash message for User class without username field
fix flash message for User class without username field

rohan07/flask-peewee,DommertTech/flask-turboduck,coleifer/flask-peewee,kashyap32/flask-peewee,coleifer/flask-peewee,coleifer/flask-peewee,kashyap32/flask-peewee,kashyap32/flask-peewee,DommertTech/flask-turboduck,rohan07/flask-peewee,DommertTech/flask-turboduck,rohan07/flask-peewee