is my enemy selector null reference a problem?

Hello Everyone,

I'm new to Unity but I'm getting along so far from the great tutorials and questions that have already been answered. I have created the script below to display the health of enemy or companion characters but which character is dependent on a Raycast from the camera (in the direction of the player) and is not initialised in void Start(). I get the "NullReferenceException" error from line 40 but it is still playable and works as intended. I just think there must be a way to code this better to avoid the error.

     CharacterHealthManager playerHealth;
     CharacterHealthManager selectedHealth;
     CanvasGroup cg;
 
     public Slider healthBar;
     public Slider selectedHealthBar;
     public Image selectedHealthBarFill;
     public Text hpText;
     public Text selectedHPText;
     public Text objName;
 
     // Use this for initialization
     void Start () {
 
         playerHealth = GameObject.FindGameObjectWithTag("Player").GetComponent<CharacterHealthManager>();
         cg = GetComponentInChildren<CanvasGroup>();
 
     }
 
     // Update is called once per frame
     void Update() {
         
         healthBar.maxValue = playerHealth.characterMaxHealth;
         healthBar.value = playerHealth.characterCurrentHealth;
         hpText.text = "HP: " + playerHealth.characterCurrentHealth + "/" + playerHealth.characterMaxHealth;
 
         DisplaySelectedHealth();
     }
 
     void DisplaySelectedHealth()
     {
 
         if (GameObject.FindGameObjectWithTag("MainCamera").GetComponent<MouseManager>().selectedObject.GetComponent<CharacterHealthManager>() != null)
         {
             cg.alpha = 1;
             selectedHealth = GameObject.FindGameObjectWithTag("MainCamera").GetComponent<MouseManager>().selectedObject.GetComponent<CharacterHealthManager>();
 
             if (selectedHealth.CompareTag("Ally"))
             {
                 selectedHealthBarFill.color = Color.magenta;
                 //TODO add Color.Lerp for smooth color transition.
             }
             else if (selectedHealth.CompareTag("Enemy"))
             {
                 selectedHealthBarFill.color = Color.black;
             }
 
             objName.text = ("" + selectedHealth.name);
             selectedHealthBar.maxValue = selectedHealth.characterMaxHealth;
             selectedHealthBar.value = selectedHealth.characterCurrentHealth;
             selectedHPText.text = "HP: " + selectedHealth.characterCurrentHealth + "/" + selectedHealth.characterMaxHealth;
         }
         else
         {
             cg.alpha = 0;
         }
 
     }
 }