AsyncMixin: Fix crash on invalid query string (PRETIXEU-AHG)

src/pretix/base/views/tasks.py

@@ -30,7 +30,9 @@ from celery import states
 from celery.result import AsyncResult
 from django.conf import settings
 from django.contrib import messages
-from django.core.exceptions import PermissionDenied, ValidationError
+from django.core.exceptions import (
+    BadRequest, PermissionDenied, ValidationError,
+)
 from django.core.files.uploadedfile import UploadedFile
 from django.db import transaction
 from django.http import HttpResponse, JsonResponse, QueryDict
@@ -131,6 +133,8 @@ class AsyncMixin:
         return data
 
     def get_result(self, request):
+        if not request.GET.get('async_id'):
+            raise BadRequest("No async_id given")
         res = AsyncResult(request.GET.get('async_id'))
         if 'ajax' in self.request.GET:
             return JsonResponse(self._return_ajax_result(res, timeout=0.25))
@@ -208,6 +212,8 @@ class AsyncAction(AsyncMixin):
 
     def get(self, request, *args, **kwargs):
         if 'async_id' in request.GET and settings.HAS_CELERY:
+            if not request.GET.get('async_id'):
+                raise BadRequest("No async_id given")
             return self.get_result(request)
         return self.http_method_not_allowed(request)