A NULL object in a List stays NULL even after initialization

Please take a look at the code below:

public class MainClass
{
    MyClass myObject1, myObject2;

    public MainClass()
    {
        myObject1 = new MyClass();
        myObject1.otherObjectsOfSameClass = new List<MyClass> { myObject2 };

        myObject2 = new MyClass();
        myObject2.otherObjectsOfSameClass = new List<MyClass> { myObject1 };
        
        myObject2.otherObjectsOfSameClass[0].TestMethod(); // works fine
        myObject1.otherObjectsOfSameClass[0].TestMethod(); // throws null reference exception
    }
}

class MyClass
{
    public List<MyClass> otherObjectsOfSameClass;

    public bool TestMethod()
    {
        return true;
    }
}

What I am trying to achieve here is to store several objects of MyClass type that hold references to the other members of the same type. When I initialize the otherObjectsOfSameClass list of each object in the MainClass constructor, I add the other object to that list. When I initialize the myObject1, the myObject2 is still null - that is expected. The list should hold a reference to myObject2, even if it's null.

However, even after I initialize myObject2, it is still null in the otherObjectsOfSameClass list of the myObject1. For some reason, the object reference in that list doesn't update. Can someone please explain this behavior? Am I misunderstanding something fundamental here?

EDIT: I realized this can be reproduced with an even simpler test code:

        List<MyClass> myList = new List<MyClass> { myObject1, myObject2 };

        myObject1 = new MyClass();

        myObject1.TestMethod(); // works fine
        myList[0].TestMethod(); // throws null reference exception

It seems that adding an uninitialized object to a list and initializing it afterwards doesn't change it in the list - even though it should be added by reference, not by value...