c# - Add element to null (empty) List Property

I got a problem.

The problem is that I try to ad an object to a list of this objects.
This list is a property, no error, but when I run it fails at this point, becouse:
"NullReferenceException". Sounds logical, becouse the Property of the list is "null", but I cant declare a property, can I?

Her is some Code snipped:

class Maps
{
    protected virtual List AllAntsAtMap { get; set; }

[...]

class Quadrangle : Maps
{
    protected override List AllAntsAtMap { get; set; }

    public override void AddAntToMap(Ant ant)
    {
        AllAntsAtMap.Add(ant);  //Error here
    }
    public override void AddAntsToMap(List ants)
    {

        foreach (Ant ant in ants)
        {
            AddAntToMap(ant);
        }
    }

[...]

Answer

Initialize the list in the constructor:

class Maps
{
    public Maps()
    {
        AllAntsAtMap = new List();
    }

    ...
}

(Since the property is declared in the superclass Maps, I'd do the initialization there rather than in the subclass Quadrangle.)