[Django]-Django UserManager create_user failing with 'NoneType' object not callable

There will be no model instance attached to that instance of MyUserManager if you instantiate it directly and call the create_user method like so. You should instead make the call from your MyUser model:

my_user = MyUser.objects.create_user(email=email, 
                                     password=password, 
                                     first_name=first_name, 
                                     last_name=last_name)