diff --git a/aqt/preferences.py b/aqt/preferences.py
index 37453d9b1..68e62f741 100644
--- a/aqt/preferences.py
+++ b/aqt/preferences.py
@@ -29,6 +29,9 @@ class Preferences(QDialog):
         self.show()
 
     def accept(self):
+        # avoid exception if main window is already closed
+        if not self.mw.col:
+            return
         self.updateCollection()
         self.updateNetwork()
         self.updateBackup()