For your solution of your error please do this...
1) put a break point of your line at this...
DataRow findRow = data.Tables["People"].Rows.Find(personID);
and press F10 then browse your findRow object if it will contain some thing or not...
then user immediate solution window and paste your line of code inside that
check this...
findRow["LastName"].ToString()
what it will give you.....
if it will give null then your code at datarow creation have something wrong and correct it there.....