Check if a GameObject has been assigned in the inspector in Unity3D 2019.3.05f

Solution 1:

Unity has a custom implementation of equality == operator (see Custom == operator, should we keep it? which often leads to confusion / unexpected behavior.

Even if an UnityEngine.Object has a value equal to null it is sometimes not == null. Object rather still stores some meta data in it e.g. you get a MissingReferenceException, not a usual NullReferenceException because Unity implemented some custom exceptions that often hold more detail to why the value is equal to null.

Especially since

public GameObject testObject = null;

is a public field it is automatically serialized and thus the null value you assigned to it will be overwritten anyway during the serialization.

UnityEngine.Object which GameObject derives from has an implicit operator bool

Does the object exist?

Instead of a direct == null comparison you should rather check for

public static bool IsNull<T>(this T myObject, string message = "") where T : UnityEngine.Object
{
    if(!myObject)
    {
        Debug.LogError("The object is null! " + message);
        return false;
    }

    return true;
}

For a general method working for any reference type you could e.g. use something like

public static bool IsNull<T>(this T myObject, string message = "") where T : class
{
    if (myObject is UnityEngine.Object obj)
    {
        if (!obj)
        {
            Debug.LogError("The object is null! " + message);
            return false;
        }
    }
    else
    {
        if (myObject == null)
        {
            Debug.LogError("The object is null! " + message);
            return false;
        }
    }

    return true;
}