Merge pull request #1921 from bagerard/error_referencefield_of_abstract_classes

Fix a bug when using a ReferenceField(AbstractClass)

mongoengine/fields.py

@@ -1176,15 +1176,6 @@ class ReferenceField(BaseField):
             self.error('You can only reference documents once they have been '
                        'saved to the database')
 
-        if (
-            self.document_type._meta.get('abstract') and
-            not isinstance(value, self.document_type)
-        ):
-            self.error(
-                '%s is not an instance of abstract reference type %s' % (
-                    value, self.document_type._class_name)
-            )
-
     def lookup_member(self, member_name):
         return self.document_type._fields.get(member_name)