title: Nullable ForeignKey field Django
body: <p>I am using django and sqlite to create a simple shop. I have a model with nullable foreign key. But when i want to add an item using admin it stops me in the foreign key field. Please help, because i don't understand why it doesn't work:</p>
  
  <pre><code>class Item(models.Model):
      id_order = models.ForeignKey(Order, null = True, default = None)
      name = models.CharField(max_length = 100)
      type_item = models.CharField(max_length = 50)
      other = models.CharField(max_length = 200)
      color = models.CharField(max_length = 100)
      cost = models.IntegerField(default = 0)
      is_available_now = models.BooleanField(default = False)
      available_count = models.IntegerField(default = 0)
      photo = models.ImageField(upload_to = "images/")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/53HHI.png" alt="enter image description here"></p>