c# - Trying to use a class as a property for other class

Usually, I only use a simple class to organize my code, but today I've tried to use a class inside another class as a property so it becomes even more organized, but I'm not being able to make it work.

This is the .cs file which contains the classes:

namespace TestZone
{
    class Item
    {
        public Category Category1 { get; set; }
        public Category Category2 { get; set; }
    }

    class Category
    {

        public string Title { get; set; }
        public string Content { get; set; }
    }
}

And this is the Program.cs file:

using System;


namespace TestZone
{
    class Program
    {
        static void Main(string[] args)
        {
            Item myItem = new Item();
            myItem.Category1.Title = "My title"; //This line throws an error.

            Console.WriteLine(myItem.Category1.Title);

            Console.ReadLine();
        }
    }
}

For some reason, the 10th line of Program.cs, where I try to assign a value to myItem.Category1.Title, this error shows up:

System.NullReferenceException: 'Object reference not set to an

instance of an object.'
TestZone.Item.Category1.get returned null.

I have also tried to put the 'Category' class inside the 'Item' class too and changed it to public like the code below, but still didn't work.

namespace TestZone
{
    class Item
    {

        public Category Category1 { get; set; }
        public Category Category2 { get; set; }


        public class Category
        {
            public string Title { get; set; }
            public string Content { get; set; }
        }
    }

}

I'm surely doing something wrong and I would appreciate if you could tell me what it is.