As I know, the person is considered as a local variable here and if the we get into the foreach block, so we have iterated the personList successfully, so person should not be null in any circumstances or any parallel scenario.
Just because you're iterating over personList
successfully doesn't mean it doesn't contain any null values. For example:
List<Person> personList = new List<Person>();
personList.Add(null);
foreach (var person in personList)
{
// Here, person will be null
}
(Additionally, if anything is modifying the list, you're generally in trouble - they're not thread-safe in the face of writers - but I don't think that needs to be part of the problem.)