bpo-39388: IDLE: Fix bug when cancelling out of configdialog (GH-18068)


Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Lib/idlelib/configdialog.py b/Lib/idlelib/configdialog.py
index 0e007b5..2f95c9c 100644
--- a/Lib/idlelib/configdialog.py
+++ b/Lib/idlelib/configdialog.py
@@ -191,6 +191,7 @@
         Methods:
             destroy: inherited
         """
+        changes.clear()
         self.destroy()
 
     def destroy(self):