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.