title: Django MPTT tree as model filter in admin
body: <p>I have a model linked to a related model that is a Django MPTT tree model, I would like to be able to filter the first model using the Django MPTT tree in the admin console.</p>
  
  <pre><code>class Tenders(models.Model):
      ...
      sector=models.ForeignKey(Sector, to_field='sectorId', null=True, blank=True,on_delete=models.CASCADE)
      ...
  
  class Sector(MPTTModel):
      name    = models.CharField(max_length = 255)
      parent  = TreeForeignKey('self', on_delete=models.CASCADE, null=True, blank=True,related_name='children')
      sectorId = models.IntegerField(default=0,null=True,unique=True)
  </code></pre>
  
  <p>In the Django admin I would like to set up the filters for the Tenders model such that the Django-MPTT tree is the filter. </p>
  
  <p>I have tried using the following: </p>
  
  <pre><code>class adminTenders(admin.ModelAdmin):    
     def linkTo(self,obj):
         return mark_safe("""&lt;a href='{}' target="_blank" &gt;Tender Link&lt;/a&gt;""".format(obj.tenderLink))
     linkTo.short_description=''
  
     list_display=(
      'title',
      'linkTo',
      'sector',
      'region',
      'repository',
      'id',
     )
     list_filter=(
      ('sector', TreeRelatedFieldListFilter),
     )
  
  admin.site.register(Tenders,adminTenders)
  </code></pre>
  
  <p>However I get the following error when trying to run this and I cant figure it out:</p>
  
  <pre><code>  File "py36/lib/python3.6/site-packages/mptt/admin.py", line 314, in field_choices
      mptt_level_indent * levels_dict[pk])
    KeyError: 0
  </code></pre>
  
  <p>Any help would be greatly appreciated. </p>
  
  <p><strong>Edit 1:</strong> I think I have isolated the issue to the fact that my foreign key in Tenders to Sectors uses a <code>to_field='sectorId</code> instead of the default to link to the <code>pk</code> column. This had to be done for backwards compatibility to an old database scheme that I am stuck with.</p>