Django-DB-Migrations: cannot ALTER TABLE because it has pending trigger events

I want to remove null=True from a TextField:

-    footer=models.TextField(null=True, blank=True)
+    footer=models.TextField(blank=True, default="")

I created a schema migration:

manage.py schemamigration fooapp --auto

Since some footer columns contain NULL I get this error if I run the migration:

django.db.utils.IntegrityError: column "footer" contains null values

I added this to the schema migration:

    for sender in orm["fooapp.EmailSender"].objects.filter(footer=None):
        sender.footer=""
        sender.save()

Now I get:

django.db.utils.DatabaseError: cannot ALTER TABLE "fooapp_emailsender" because it has pending trigger events

What is wrong?

Django-DB-Migrations: cannot ALTER TABLE because it has pending trigger events __del__: Questions

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

from time import sleep
sleep(0.1) # Time in seconds

Django-DB-Migrations: cannot ALTER TABLE because it has pending trigger events __del__: Questions