Return item with the index.

        If the index is negative number or out of the list, return None.

def item(self, index: int) -> Optional[Node]:
        """Return item with the index.

        If the index is negative number or out of the list, return None.
        """
        if not isinstance(index, int):
            raise TypeError(
                'Indeces must be integer, not {}'.format(type(index)))
        return self.__nodes[index] if 0 <= index < self.length else None