Fixed #31870 -- Fixed crash when populating app registry with empty or without apps module.

Regression in 3f2821af6b.

django/apps/config.py

@@ -89,6 +89,7 @@ class AppConfig:
         """
         # create() eventually returns app_config_class(app_name, app_module).
         app_config_class = None
+        app_config_name = None
         app_name = None
         app_module = None
 
@@ -161,12 +162,15 @@ class AppConfig:
                     )
                 else:
                     message += (
-                        "However, Django's automatic detection picked another "
+                        "However, Django's automatic detection %s. You should "
-                        "configuration, %r. You should move the default "
+                        "move the default config class to the apps submodule "
-                        "config class to the apps submodule of your "
+                        "of your application and, if this module defines "
-                        "application and, if this module defines several "
+                        "several config classes, mark the default one with "
-                        "config classes, mark the default one with default = "
+                        "default = True." % (
-                        "True." % app_config_name
+                            "picked another configuration, %r" % app_config_name
+                            if app_config_name
+                            else "did not find this configuration"
+                        )
                     )
                 warnings.warn(message, RemovedInDjango41Warning, stacklevel=2)
                 entry = new_entry

tests/apps/explicit_default_config_empty_apps/__init__.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+default_app_config = 'apps.explicit_default_config_empty_apps.ExplicitDefaultConfigEmptyApps'
+
+
+class ExplicitDefaultConfigEmptyApps(AppConfig):
+    name = 'apps.explicit_default_config_empty_apps'

tests/apps/explicit_default_config_without_apps/__init__.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+default_app_config = 'apps.explicit_default_config_without_apps.ExplicitDefaultConfigWithoutApps'
+
+
+class ExplicitDefaultConfigWithoutApps(AppConfig):
+    name = 'apps.explicit_default_config_without_apps'

tests/apps/tests.py

@@ -10,9 +10,13 @@ from django.test.utils import extend_sys_path, isolate_apps
 from django.utils.deprecation import RemovedInDjango41Warning
 
 from .explicit_default_config_app.apps import ExplicitDefaultConfig
+from .explicit_default_config_empty_apps import ExplicitDefaultConfigEmptyApps
 from .explicit_default_config_mismatch_app.not_apps import (
     ExplicitDefaultConfigMismatch,
 )
+from .explicit_default_config_without_apps import (
+    ExplicitDefaultConfigWithoutApps,
+)
 from .models import SoAlternative, TotallyNormal, new_apps
 from .one_config_app.apps import OneConfig
 from .two_configs_one_default_app.apps import TwoConfig
@@ -520,3 +524,51 @@ class DeprecationTests(SimpleTestCase):
             with self.settings(INSTALLED_APPS=['apps.explicit_default_config_mismatch_app']):
                 config = apps.get_app_config('explicit_default_config_mismatch_app')
             self.assertIsInstance(config, ExplicitDefaultConfigMismatch)
+
+    def test_explicit_default_app_config_empty_apps(self):
+        """
+        Load an app that specifies a default AppConfig class in __init__ and
+        have an empty apps module.
+        """
+        msg = (
+            "'apps.explicit_default_config_empty_apps' defines "
+            "default_app_config = 'apps.explicit_default_config_empty_apps."
+            "ExplicitDefaultConfigEmptyApps'. However, Django's automatic "
+            "detection did not find this configuration. You should move the "
+            "default config class to the apps submodule of your application "
+            "and, if this module defines several config classes, mark the "
+            "default one with default = True."
+        )
+        with self.assertRaisesMessage(RemovedInDjango41Warning, msg):
+            with self.settings(INSTALLED_APPS=['apps.explicit_default_config_empty_apps']):
+                pass
+        with ignore_warnings(category=RemovedInDjango41Warning):
+            with self.settings(INSTALLED_APPS=['apps.explicit_default_config_empty_apps']):
+                self.assertIsInstance(
+                    apps.get_app_config('explicit_default_config_empty_apps'),
+                    ExplicitDefaultConfigEmptyApps,
+                )
+
+    def test_explicit_default_app_config_without_apps(self):
+        """
+        Load an app that specifies a default AppConfig class in __init__ and do
+        not have an apps module.
+        """
+        msg = (
+            "'apps.explicit_default_config_without_apps' defines "
+            "default_app_config = 'apps.explicit_default_config_without_apps."
+            "ExplicitDefaultConfigWithoutApps'. However, Django's automatic "
+            "detection did not find this configuration. You should move the "
+            "default config class to the apps submodule of your application "
+            "and, if this module defines several config classes, mark the "
+            "default one with default = True."
+        )
+        with self.assertRaisesMessage(RemovedInDjango41Warning, msg):
+            with self.settings(INSTALLED_APPS=['apps.explicit_default_config_without_apps']):
+                pass
+        with ignore_warnings(category=RemovedInDjango41Warning):
+            with self.settings(INSTALLED_APPS=['apps.explicit_default_config_without_apps']):
+                self.assertIsInstance(
+                    apps.get_app_config('explicit_default_config_without_apps'),
+                    ExplicitDefaultConfigWithoutApps,
+                )