diff --git a/qt/aqt/taskman.py b/qt/aqt/taskman.py
index 6f976838b..fa103199f 100644
--- a/qt/aqt/taskman.py
+++ b/qt/aqt/taskman.py
@@ -73,7 +73,8 @@ class TaskManager(QObject):
 
         def wrapped_done(fut):
             self.mw.progress.finish()
-            on_done(fut)
+            if on_done:
+                on_done(fut)
 
         self.run_in_background(task, wrapped_done)