Fix EventPluginSignal to recognize Django apps with custom app labels

src/pretix/base/signals.py

@@ -8,6 +8,15 @@ from django.dispatch.dispatcher import NO_RECEIVERS
 
 from .models import Event
 
+app_cache = {}
+
+
+def _populate_app_cache():
+    global app_cache
+    apps.check_apps_ready()
+    for ac in apps.app_configs.values():
+        app_cache[ac.name] = ac
+
 
 class EventPluginSignal(django.dispatch.Signal):
     """
@@ -30,18 +39,16 @@ class EventPluginSignal(django.dispatch.Signal):
         if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
             return responses
 
+        if not app_cache:
+            _populate_app_cache()
+
         for receiver in self._live_receivers(sender):
             # Find the Django application this belongs to
             searchpath = receiver.__module__
-            app = None
             mod = None
             while True:
-                try:
-                    if apps.is_installed(searchpath):
-                        app = apps.get_app_config(searchpath.split(".")[-1])
-                except LookupError:
-                    pass
-                if "." not in searchpath:
+                app = app_cache.get(searchpath)
+                if "." not in searchpath or app:
                     break
                 searchpath, mod = searchpath.rsplit(".", 1)