Returns the Node at given index.

        :param index: Index.
        :type index: QModelIndex
        :return: Node.
        :rtype: AbstractCompositeNode or GraphModelNode

def get_node(self, index):
        """
        Returns the Node at given index.

        :param index: Index.
        :type index: QModelIndex
        :return: Node.
        :rtype: AbstractCompositeNode or GraphModelNode
        """

        if not index.isValid():
            return self.__root_node
        return index.internalPointer() or self.__root_node