Fixed #27967 -- Fixed KeyError in admin's inline form with inherited non-editable pk.

Thanks Robin Anupol for the initial report and workaround.

tests/admin_inlines/models.py

@@ -56,6 +56,10 @@ class NonAutoPKBook(models.Model):
         super().save(*args, **kwargs)
 
 
+class NonAutoPKBookChild(NonAutoPKBook):
+    pass
+
+
 class EditablePKBook(models.Model):
     manual_pk = models.IntegerField(primary_key=True)
     author = models.ForeignKey(Author, models.CASCADE)