peewee cannot insert data after new column insertion

I am using peewee module for managing data in Sqlite database. My use case scenarios is that I will be creating a database with certain fields. I also need to add columns into the existing database at certain time. Below is my code that is supposed to work as expected:

from peewee import *
import os
from playhouse.migrate import *

my_db = SqliteDatabase('my_database.db')
migrator = SqliteMigrator(my_db)

class FirstTable(Model):
    first_name = CharField(null=True)
    last_name = CharField(null=True)

    class Meta:
        database = my_db

class Checkit:

    def __init__(self):
        self.db = my_db
        self.migrator = migrator

    def makeDatabse(self):
        if os.path.exists("my_database.db"):
            print "File Exists remove it"
            os.remove("my_database.db")
        try:
            self.db.connect()
            self.db.create_tables([FirstTable,])
        except OperationalError:
            print "Table Exists"

    def insertDatas(self):
        with self.db.atomic():
            for i in range(10):
                first_name_ = "Hello " + str(i)
                last_name_ = "World " + str(i)
                db_ = FirstTable(first_name=first_name_, last_name = last_name_)
                db_.save()

    def alterDatabase(self, columns):
        with self.db.transaction():
            columnField = CharField(null=True)
            for column in columns:              
                migrate(migrator.add_column("firsttable", column, columnField))

    def insertAfterAlteringDatabase(self):
        with self.db.atomic():
            for i in range(20,30):
                first_name_ = "Hello " + str(i)
                last_name_ = "World " + str(i)
                address_ = "Address " + str(i)
                db_ = FirstTable(first_name=first_name_, last_name = last_name_, address=address_)
                db_.save()

ch = Checkit() 
ch.makeDatabse()
ch.insertDatas()
ch.alterDatabase(["address"])
ch.insertAfterAlteringDatabase()

After adding a new column address for which null=True, I am doing some insertion into the altered database. I am expected to see Address data into address field, but I am not getting any of these data. Instead it's NULL. My code should have worked fine, but its not working as expected. Whats the problem here?