Multi export: Allow restriction to organizer

src/pretix/base/services/export.py

@@ -50,6 +50,8 @@ def multiexport(organizer: Organizer, user: User, fileid: str, provider: str, fo
         responses = register_multievent_data_exporters.send(organizer)
 
         for receiver, response in responses:
+            if not response:
+                continue
             ex = response(events)
             if ex.identifier == provider:
                 d = ex.render(form_data)

src/pretix/control/views/organizer.py

@@ -1145,7 +1145,7 @@ class ExportMixin:
             organizer=self.request.organizer
         )
         responses = register_multievent_data_exporters.send(self.request.organizer)
-        for ex in sorted([response(events) for r, response in responses], key=lambda ex: str(ex.verbose_name)):
+        for ex in sorted([response(events) for r, response in responses if response], key=lambda ex: str(ex.verbose_name)):
             if self.request.GET.get("identifier") and ex.identifier != self.request.GET.get("identifier"):
                 continue