Initializes the Widget ui.

def __initialize_ui(self):
        """
        Initializes the Widget ui.
        """

        self.setAutoScroll(True)
        self.setSelectionMode(QAbstractItemView.ExtendedSelection)
        self.setIndentation(self.__tree_view_indentation)
        self.setDragDropMode(QAbstractItemView.DragOnly)
        self.setHeaderHidden(True)

        self.__set_default_ui_state()

        # Signals / Slots.
        self.model().modelReset.connect(self.__set_default_ui_state)