Fix a crash when the help button is clicked

- This was to do with the way the presenter is created as an optional. It doesn't need to be an optional here - a unique pointer is better and makes ownership clearer.
- This commit also includes some tidying of the MainWindowSubscriber which was being used incorrectly.
- It also includes replacing some calls to m_presenter with calls to m_notifyee. They are the same object but we should use m_notifyee to be more in keeping with MVP. There is one remaining use of m_presenter in closeEvent but it is beyond the scope of this PR to fix that.

Re #23027