The lifespan of an object instance in a Monobehavior script

A MonoBehavior script ReferenceAgent.cs has a instance of non-MonoBehavior class ReferenceObject. ReferenceObject.cs has a random string attribute which is for testing purpose.

Now we have a MonoBehaivor script Test.cs, which has references of ReferenceAgent component and its ReferenceObject instance.

If I destroy the GameObject - the one ReferenceAgent attached - in Test.cs, I can't visit the reference of ReferenceAgent anymore, but I can still visit the reference of ReferenceObject which should be an attribute in ReferenceAgent.

Is this a stable feature or just because I'm visiting the attribute which causing the program keep the instance in memory?

 public class ReferenceAgent : MonoBehaviour {
 
     private ReferenceObject refObject;
 
     public ReferenceObject RefObject
     {
         get { return refObject; }
     }
 
     // Use this for initialization
     void Awake () {
         refObject = new ReferenceObject(Random.Range(100, 999).ToString());
     }
 }

============================================================

 public class ReferenceObject {
 
     private string testObjName;
 
     public string TestObjName
     {
         get { return testObjName; }
     }
 
     public ReferenceObject(string name)
     {
         this.testObjName = name;
     }
 }

============================================================

 public class Test : MonoBehaviour {
 
     public ReferenceAgent agent;
     public ReferenceObject refObject;
 
     // Use this for initialization
     void Start () {
         refObject = agent.RefObject;
     }
 
     void Update() {
         if(Input.GetKeyDown(KeyCode.Q))
         {
             Debug.Log(refObject.TestObjName); // This will work after pressing key W
             Debug.Log(agent);
         }
 
         if(Input.GetKeyDown(KeyCode.W))
         {
             Destroy(agent.gameObject);
         }
     }
 }