Resolve "TaskUserAssignment matching query does not exist."

Closes #747 (closed)

aleksis/core/tasks.py

@@ -66,6 +66,11 @@ def send_notification_for_done_task(task_id):
     # Wait five seconds to ensure that the client has received the final status
     time.sleep(5)
 
-    assignment = TaskUserAssignment.objects.get(task_result__task_id=task_id)
+    try:
+        assignment = TaskUserAssignment.objects.get(task_result__task_id=task_id)
+    except TaskUserAssignment.DoesNotExist:
+        # No foreground task
+        return
+
     if not assignment.result_fetched:
         assignment.create_notification()