def check_wide_data_for_blank_choices(choice_col, wide_data):
    """
    Checks `wide_data` for null values in the choice column, and raises a
    helpful ValueError if null values are found.

    Parameters
    ----------
    choice_col : str.
        Denotes the column in `wide_data` that is used to record each
        observation's choice.
    wide_data : pandas dataframe.
        Contains one row for each observation. Should contain `choice_col`.

    Returns
    -------
    None.
    """
    if wide_data[choice_col].isnull().any():
        msg_1 = 'One or more of the values in wide_data[choice_col] is null.'
        msg_2 = ' Remove null values in the choice column or fill them in.'
        raise ValueError(msg_1 + msg_2)
    return None
