Return a widget for the line number passed.

def _get_at_pos(self, pos):
        """Return a widget for the line number passed."""

        if pos < 0:
            # line 0 is the start of the file, no more above
            return None, None

        if len(self.lines) > pos:
            # we have that line so return it
            return self.lines[pos], pos

        if self.file is None:
            # file is closed, so there are no more lines
            return None, None

        assert pos == len(self.lines), "out of order request?"

        self.read_next_line()

        return self.lines[-1], pos