Give the class a constructor and set the variable there
public class PartnerProjectVM
{
public int idPartnerProject { get; set; }
public string ProjectName { get; set; }
public string ProjectDescription { get; set; }
public virtual List<PartnerProjectVM> listPartnerProjectVM { get; set; }
public PartnerProjectVM ()
{
this.listPartnerProjectVM = new List<PartnerProjectVM>()
}
}