Why is FieldInfo.GetValue(null) not working in static constructor

See the code below. I want a class that automatically enumerates all the defined static readonly instances of its own type (see TestClass as an example, it defines 3 static readonly instances of its own type).

I want this automation because I want to loop over the defined types and not risk the change of forgetting to add a new instance to the list of All.

Ok, I have it working, that is not the point. But why doesn't FillAll work when called from a static constructor? See the commented static constructor in DefinedInstancesBase<T> code. I mean FieldInfo.GetValue(null) returns null in the static constructor, though the debugger has already hit creating the static readonly instances before the FieldInfo.GetValue(null) is called.

I'm very curious why it doesn't work. Is this by design?

public abstract class DefinedInstancesBase<T>
{
    public static IList<T> All
    {
        get
        {
            if (_All == null)
            {
                FillAll();
            }
            return _All;
        }
    }

    //Why this doesn't work? No idea.
    //static DefinedInstances()
    //{
    //    FillAll();
    //}

    private static void FillAll()
    {
        var typeOfT = typeof(T);
        var fields = typeOfT.GetFields(BindingFlags.Public | BindingFlags.Static);
        var fieldsOfTypeT = fields.Where(f => f.FieldType == typeOfT);
        _All = new List<T>();
        foreach (var fieldOfTypeT in fieldsOfTypeT)
        {
            _All.Add((T)fieldOfTypeT.GetValue(null));
        }
    }

    private static List<T> _All = null;
}

[TestClass]
public class DefinedInstancesTest
{
    [TestMethod]
    public void StaticReadOnlyInstancesAreEnumerated()
    {
        //Given
        var expectedClasses = new List<TestClass>
        {
            TestClass.First,
            TestClass.Second,
            TestClass.Third,
        };

        //When
        var actualClasses = TestClass.All;

        //Then
        for (var i=0; i<expectedClasses.Count; i++)
        {
            Assert.AreEqual(expectedClasses[i].Id, actualClasses[i].Id);
        }
    }

    private class TestClass : DefinedInstancesBase<TestClass>
    {
        public static readonly TestClass First = new TestClass(1);
        public static readonly TestClass Second = new TestClass(2);
        public static readonly TestClass Third = new TestClass(3);

        public int Id { get; private set; }

        private TestClass(int pId)
        {
            Id = pId;
        }
    }
}