Self Reference and TextMesh inside of a script to change the text

I try to implement a health system and want to refere from my script HealthHandle wich is attached to the health prefab, to the TextMesh and change it to something else. The problem is when i do this i get the exception that the object is not assigned.

     TextMesh tm;
 
     // Use this for initialization
     void Start () {
         tm = gameObject.GetComponent<TextMesh>();
     }
 
     public void decrease ()
     {
         if (current() > 1)
         {
             tm.text = tm.text.Remove(tm.text.Length - 1);
         }
         else
         {
             Destroy(transform.parent.gameObject);
         }
     }

what im doing wrong here?

the function decrease gets called from another script like this:

     public HealthHandle healthHandle; 
 
         healthHandle.decrease();

the Object that Contains the HealthHandle is atached in the unity editor