title: Django-Markdownx MarkdownxModelAdmin template error
body: <p>I tried to test Django-Markdownx in my django project, but it keeps makes error. Here is my models.py.</p>
  
  <pre><code>from django.db import models
  from django.utils import timezone
  
  from markdownx.models import MarkdownxField
  
  # Create your models here.
  
  class Article(models.Model):
  title = models.CharField(max_length=200)
  author = models.ForeignKey('auth.User', on_delete=models.CASCADE)
  article = MarkdownxField()
  created_date = models.DateTimeField(default=timezone.now)
  published_date = models.DateTimeField(blank=True, null=True)
  
  def publish(self):
      self.published_date = timezone.now
      self.save()
  
  def __str__(self):
      return self.title
  </code></pre>
  
  <p>And, here is my admin.py.</p>
  
  <pre><code>from django.db import models
  from django.contrib import admin
  from markdownx.admin import MarkdownxModelAdmin
  
  from .models import Article
  
  # Register your models here.
  admin.site.register(Article, MarkdownxModelAdmin)
  </code></pre>
  
  <p>I did as docs example said, but it keeps make an error like below, when I tried to select any Article objects in admin page.</p>
  
  <p><a href="https://i.stack.imgur.com/xoJyl.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>I confirmed that if I change the Admin.admin to TextField(), it works properly.</p>
  
  <p>Need help..</p>