Need help in implementing index in interface with multiple properties

Question

I have just started learning C#, and I am trying to a implement an interface with indexing properties that looks like this:

public interface ITest<T>
{
    T this[int index] {get;}
    int PropertyA {get;}
    int PropertyB {get;}
}

public interface IProperties
{
    float PropertyC {get;set;}
    float PropertyD {get;set;}
}

public class Program : ITest<IProperties>
{
    public int PropertyA => 3;
    public int PropertyB => 1;
    public float[] Temps = new float[10]
    {
        21, 22, 23, 24, 25, 26, 27, 28, 29, 30
    };
    private IProperties data;
    public IProperties this[int index]
    {
        get
        {
            data.PropertyC = Temps[index];
            data.PropertyD = Temps[index + 1];
            return data[index];
        }
        set
        {
             data = value;
        }
    }

    static void Main(string[] args)
    {
        Program program = new Program();
        Console.WriteLine(program.PropertyA);
        Console.WriteLine(program.PropertyB);
        Console.WriteLine(program.Temps[0]);
        Console.WriteLine(program[0].PropertyC); // expect 21
        Console.WriteLine(program[0].PropertyD); // expect 22
    }
}

At the end, I keep getting null exception in trying to get the value of program[0].PropertyC. Where did it go wrong, and what shall I do to make it right?

**UPDATE: As much as I know that data is a null and need to be instantiated, I need some ideas on how to instantiate data as it is a interface type. Or is there any other way to get program[0].PropertyC so data wouldn't be used at the first place?