NullReferenceException when changing Canvas text via code

Hi. I've been trying to add a health counter to my game, by changing canvas text via code. I get a NullReferenceException whenever I use it though. This is the code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class UIScript : MonoBehaviour {
     public Text HealthText;
     int PrevHealth;
     // Use this for initialization
     void Start () {
         HealthText.text = "Health: " + GameObject.Find("FPSController").GetComponent<PlyHealthScript>().curHealth;
 
         HealthText = GameObject.Find("FPSController").GetComponent<Text>();
 
         PrevHealth = GameObject.Find("FPSController").GetComponent<PlyHealthScript>().curHealth;
 
 
     }
 
     // Update is called once per frame
     void Update () {
 
 
 
     }
 }

Here is the error:

 NullReferenceException: Object reference not set to an instance of an object
 UIScript.Start () (at Assets/Scripts/UIScript.cs:11)

Can anyone help? It's been driving me crazy.