Preferences dialog stops working "after some time"

[ffissore]

If you click "configure", then close the window, then click "preferences", the close the window, a few times, at some point "preferences" dialogue won't show up any more and a python exception about nonetype not having foreach method will be logged

[ffissore]

Update: it suffices to click "preferences", then close the window pressing the Esc key, then click again on "preferences"

[ffissore]

This looks like a ubuntu 18 only problem: just tried on debian buster and it just works