Django – AttributeError: 'User' object has no attribute 'fullclean'

I am working on a Django project on the IntelliJ IDEA IDE. I am writing unit tests in the test.py file for the User model class. When running the test.py file with the command python manage.py test it throws 2 errors on the test_username_cannot_be_blank method and the test_valid_user method. The error is the same for both methods

AttributeError: ‘User’ object has no attribute ‘fullclean’.

IDEA is also giving me a Instance attribute user defined outside init warnig on line 9 of the test.py file ("self.user = User.objects.create_user()") is where the compiler warning occurs.

What exactly is causing of the AttributeError and the compiler warning?

# test.py file
from django.test import TestCase
from django.core.exceptions import ValidationError
from .models import User


class UserModelTestCase(TestCase):
    def setUp(self):
        self.user = User.objects.create_user(
            '@johndoe',
            first_name='John',
            last_name='Doe',
            email='[email protected]',
            password='Password123',
            bio='The quick brown fox jumps over the lazy dog.'
        )

    def test_valid_user(self):
        self._assert_user_is_valid()

    def test_username_cannot_be_blank(self):
        self.user.username = ''
        self._assert_user_is_invalid()

    def _assert_user_is_valid(self):
        try:
            self.user.fullclean()
        except ValidationError:
            self.fail('user should be valid')

    def _assert_user_is_invalid(self):
        with self.assertRaises(ValidationError):
            self.user.fullclean()

# models.py file
from django.db import models
from django.contrib.auth.models import AbstractUser


class User(AbstractUser):
    bio = models.TextField()

Looked on stackoverflow but could’nt find any solution to the problem.