GUI: possible issue with mapper after refactoring

In file ParameterTuningDelegate.cpp in function

QWidget* ParameterTuningDelegate::createEditor(QWidget* parent, const QStyleOptionViewItem& option,
                                               const QModelIndex& index) const

an old comment says about some problem:

// FIXME there is an issue with time of life of editor .vs. item

Probably it is about something occurred after removing mapper and SessionItem.

This should be cheked.