added errorchecking for get_group

[WillySong]

I can't believe it took me all night to understand and debug this.
I have no clue how to do the rest of this below.

• closes dataframe.groupby incorrect with multiindex and None value pandas-dev/pandas#32492
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[WillySong]

For reference, the error truly exists in how we use _get_indices. I cannot read nor debug whatever god awful C24 lambda code they wrote for that function, but it returns what we see as a valid input in our get_group function. I have added error checking this case (as per the issue). Please verify this change works (two test cases provided if you run the file) and then I'll build some more test cases.

[letapxad]

For reference, the error truly exists in how we use _get_indices. I cannot read nor debug whatever god awful C24 lambda code they wrote for that function, but it returns what we see as a valid input in our get_group function. I have added error checking this case (as per the issue). Please verify this change works (two test cases provided if you run the file) and then I'll build some more test cases.

I can agree about the _get_indices implementation

[julesyan]

Tests dont fail on the original code, need more cases.
Fix does not complete original issue after testing in dev environment.