How to populate list as class object?

How do you populate a list as a class object? For example, this does not work:

[DataContract]
public class JsonReviewFormFields
{
    [DataMember]
    public PersonalDevelopmentPlan personalDevelopmentPlan { get; set; }      
}

public class PersonalDevelopmentPlan
{
    public List<ShortTerm> shortTerm { get; set; }
    public List<LongTerm> longTerm { get; set; }
}

public class ShortTerm
{
    public string workRelated { get; set; }
    public string structured { get; set; }
    public string informal   { get; set; }       
    public string reviewDate { get; set; }
}

public class LongTerm
{
    public string workRelated { get; set; }
    public string structured { get; set; }
    public string informal { get; set; }
    public string reviewDate { get; set; }   
}

This is controller action:

 public JsonReviewFormFields GetReviewForm()
    {


        PersonalDevelopmentPlan personalDevelopmentPlan = new PersonalDevelopmentPlan();

         List<ShortTerm> _itemsShort = new List<ShortTerm>();
        _itemsShort.Add(new ShortTerm { workRelated = "workRelated text", structured = "structured text", informal = "informal text", reviewDate = "reviewDate" });
        jsonReviewFormFields.personalDevelopmentPlan.shortTerm =  _itemsShort;

        List<LongTerm> _itemsLong = new List<LongTerm>();
        _itemsLong.Add(new LongTerm { workRelated = "workRelated text", structured = "structured text", informal = "informal text", reviewDate = "reviewDate" });
        jsonReviewFormFields.personalDevelopmentPlan.longTerm =  _itemsLong;     

        return jsonReviewFormFields;

    }

}

The code crashes at .shortTerm = . It's probably a basic object orientated error. How do you populate the list?