[1.10.x] Fixed #26686 -- Fixed crash when registering model signals with abstract senders.

Backport of 08014fe75b from master

tests/signals/tests.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from django.apps.registry import Apps
 from django.db import models
 from django.db.models import signals
 from django.dispatch import receiver
@@ -317,3 +318,14 @@ class LazyModelRefTest(BaseSignalTest):
 
         Created()
         self.assertEqual(received, [])
+
+    def test_register_model_class_senders_immediately(self):
+        """
+        Model signals registered with model classes as senders don't use the
+        Apps.lazy_model_operation() mechanism.
+        """
+        # Book isn't registered with apps2, so it will linger in
+        # apps2._pending_operations if ModelSignal does the wrong thing.
+        apps2 = Apps()
+        signals.post_init.connect(self.receiver, sender=Book, apps=apps2)
+        self.assertEqual(list(apps2._pending_operations), [])