Referencing an interface from another script

Hi, everyone. I'm still very green in C# and Unity, and I was hoping some of you could help me with an issue I ran into while making my first game.

There's 9 GameObjects that the player can click on. When one of them is clicked, Shoot(); is executed and it calls a Coroutine called Growth(); which then modifies its scale.

 public void Shoot()                                                                              
     {
 
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         RaycastHit hit;
 
         if (Physics.Raycast(ray, out hit) == GetComponent<TreeGrower>())
         {
             //Debug.Log(hit.transform.name);
             StartCoroutine(myTreeGrower.Growth(15));
         }
 
     }

Now, Growth(); is in a second script (myTreeGrower), and I can't manage to call it from the 'main' script. The error that pops up is "Object reference not set to an instance of an object", which I believe means that the script is simply not attached to a GameObject, but I can't create a reference in-script so that I can then attach it in Unity's editor.

 public  IEnumerator Growth(float time)
     {
         Vector3 initialTreeScale = tree.transform.localScale;
         Vector3 finalTreeScale = new Vector3(0.70f, 3f, 0.70f);
 
         float currentTime = 0.0f;
 
         do
         {
             tree.transform.localScale = Vector3.Lerp(initialTreeScale, finalTreeScale, currentTime / time);
             currentTime += Time.deltaTime;
             yield return null;
 
         } while (currentTime <= time);
 
     }
 

The second issue is that, when I had Growth(); in the same script (before I created the second one), I couldn't get it to be implemented in the object that was clicked. I could click any of the 9, but every time only one of them would be affected. Always the same one.

Is there a way to determine which object was clicked, and then modify that object's scale?

All things aside, if there's anything you see I can do better to improve my coding, please let me know.

Thank you all in advance!