[4.0.x] Fixed #33547 -- Fixed error when rendering invalid inlines with readonly fields in admin.

Regression in de95c826673be9ea519acc86fd898631d1a11356.

Thanks David Glenck for the report.
Backport of 445b075def2c037b971518963b70ce13df5e88a2 from main

2 files changed, 20 insertions, 0 deletions

diff --git a/tests/admin_inlines/models.py b/tests/admin_inlines/models.py
index 47c5b91828..a8d2ee02e1 100644
--- a/tests/admin_inlines/models.py
+++ b/tests/admin_inlines/models.py
@@ -5,6 +5,7 @@ import random
 
 from django.contrib.contenttypes.fields import GenericForeignKey
 from django.contrib.contenttypes.models import ContentType
+from django.core.exceptions import ValidationError
 from django.db import models
 
 
@@ -204,6 +205,9 @@ class Question(models.Model):
     text = models.CharField(max_length=40)
     poll = models.ForeignKey(Poll, models.CASCADE)
 
+    def clean(self):
+        raise ValidationError("Always invalid model.")
+
 
 class Novel(models.Model):
     name = models.CharField(max_length=40)
diff --git a/tests/admin_inlines/tests.py b/tests/admin_inlines/tests.py
index 3bd3c881d4..f61ed1aeb2 100644
--- a/tests/admin_inlines/tests.py
+++ b/tests/admin_inlines/tests.py
@@ -241,6 +241,22 @@ class TestInline(TestDataMixin, TestCase):
         # column cells
         self.assertContains(response, "<p>Callable in QuestionInline</p>")
 
+    def test_model_error_inline_with_readonly_field(self):
+        poll = Poll.objects.create(name="Test poll")
+        data = {
+            "question_set-TOTAL_FORMS": 1,
+            "question_set-INITIAL_FORMS": 0,
+            "question_set-MAX_NUM_FORMS": 0,
+            "_save": "Save",
+            "question_set-0-text": "Question",
+            "question_set-0-poll": poll.pk,
+        }
+        response = self.client.post(
+            reverse("admin:admin_inlines_poll_change", args=(poll.pk,)),
+            data,
+        )
+        self.assertContains(response, "Always invalid model.")
+
     def test_help_text(self):
         """
         The inlines' model field help texts are displayed when using both the