on_delete=SET_NULL 설정시 ERROR 발생

from django.contrib.auth.models import User from django.db import models # Create your models here. class Article(models.Model): writer = models.ForeignKey(User, on_delete=models.SET_NULL, related_name='arrticle') title = models.CharField(max_length=200, null=True) image = models.ImageField(upload_to='article/', null=False) content = models.TextField(null=True) created_at = models.DateField(auto..