Save Script and use it causes Errors

 chosenBombType = GameObject.FindWithTag("Player").GetComponent<FirstPersonController>().GetBombType();
 
 
 hScript = GameObject.FindGameObjectWithTag("Player").GetComponent<Health_Script>();

somethimes i get errors with this 2 lines of code. Sometimes its works and than when i chance something in the scripts nothing todo with this lines it get an error. I dont understand why i cant save the script of an gameobject and use it after that?