Clear the new record for Preference

The Preference window create a new record to simulate the read using
preferences values. But this mark the record has modified and prevent to click
on buttons. So the new record must be canceled just after "new" call.

issue5663
review28351002

tryton/gui/window/preference.py

@@ -65,6 +65,7 @@ class Preference(NoModal):
             self.win.destroy()
             self.win = None
             return
+        self.screen.current_record.cancel()
         self.screen.current_record.set(preferences)
         self.screen.current_record.id = rpc._USER
         self.screen.current_record.validate(softvalidation=True)