flask-admin TypeError: create_view() got an unexpected keyword argument 'cls'

TypeError: create_view() got an unexpected keyword argument 'cls'

class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)
    first_name = db.Column(db.String(255))
    last_name = db.Column(db.String(255))
    email = db.Column(db.String(255), unique=True)
    password = db.Column(db.String(255))
    active = db.Column(db.Boolean())
    confirmed_at = db.Column(db.DateTime())
    roles = db.relationship('Role', secondary=roles_users,
                            backref=db.backref('users', lazy='dynamic'))

    def __str__(self):
        return self.email

        # 返回值为空，或者，类型不是str flask-admin会报错，TypeError: __str__ returned non-string (type NoneType)

        # During handling of the above exception, another exception occurred:

        # TypeError: create_view() got an unexpected keyword argument 'cls'