Object reference unassigning itself at runtime in UNITY

I'm trying to reference the grid object which contains a script that has my inventorygrid class. I'm referencing from another script attached to the ui camera.

Its assigned like this:

I'm just trying to reference it like this:

    public class InventoryController : MonoBehaviour
{
    [SerializeField] private InventoryGrid selectedInventoryGrid;
 
    private void Update()
    {
        if (selectedInventoryGrid = null)
        {
            Debug.Log("InventoryGrid IS null.");
            return;
        }
 
 
        selectedInventoryGrid.GetTileGridPosition(Input.mousePosition);
    }
}

However, when trying to run the referenced class and method, I get "Object reference not set to an instance of an object.". I've noticed that the "Selected Inventory Grid" that I've assigned just unassigns itself when I run the game.

selectedInventoryGrid = null

It should be

selectedInventoryGrid == null

I was setting it null without realising.

You have two issues. First is already mentioned by @DeveloperLewis.

 if (selectedInventoryGrid = null)
    {
        Debug.Log("InventoryGrid IS null.");
        return;
    }

If you are checking for null, it should be "==" not "=".

Second issue is this field:

    [SerializeField] private InventoryGrid selectedInventoryGrid;

I think that some versions of Unity have a bug (or it worked all the time like this?) that they drop the reference if your field is PRIVATE but has [SerializeField] attribute assigned to it.

Solution is that, you need to make that field PUBLIC.

public InventoryGrid selectedInventoryGrid;