Return the capture board or None if can't find it.

def get_capture(self):
        """Return the capture board or None if can't find it."""
        # game
        game_image = self._game_image_from_screen('capture')
        if game_image is None:
            return
        # board
        board = self._board_from_game_image(game_image)
        if board is None:
            return
        if board.is_empty():
            return
        return board