Upon clicking: Object reference not set to an instance of an object

Hello,

In a tower defense game, I created a healthbar that would float above my enemy objects, using some rectangles with OnGUI(). Everything works as I would expect, except for the fact that when I click anywhere on the screen while it is running, I get an influx of this error:

NullReferenceException: Object reference not set to an instance of an object, Healthbar.OnGUI() (at Healthbar.cs:19)

It seems bizarre to me. I have objects assigned to both of the public fields (the textures), and I don't even have any functionality programmed in that would react to clicking in any way. Is there some sort of hidden click functionality while in-game in Unity that I am unaware of?

My healthbar code:

 using UnityEngine;
 using System.Collections;
 
 public class Healthbar : MonoBehaviour {
 
     public Texture2D bgTexture;
     public Texture2D healthbarTexture;
 
     private GUIStyle healthStyle;
     private Vector2 size = new Vector2(32,8);
     private int rawPercent = 100;
     
     void Start () {
         healthStyle = new GUIStyle();
         healthStyle.border = new RectOffset(2,2,2,2);
     }
     
     void OnGUI() {
         Vector3 pos =
             Camera.current.WorldToScreenPoint(transform.position);
         pos.y = Screen.height - (pos.y + 1);
 
         GUI.BeginGroup(new Rect(pos.x - 4, pos.y, size.x, size.y));
             GUI.Box(new Rect(0,0, size.x, size.y), bgTexture, healthStyle);
             float truePercent = rawPercent / 100f;
             GUI.BeginGroup(new Rect(0,0, (float)size.x * truePercent, size.y));
                 GUI.Box(new Rect(0,0, size.x, size.y), healthbarTexture, healthStyle);
             GUI.EndGroup();
         GUI.EndGroup();
     }
     
     public void AdjustHealth(int damage) {
         rawPercent -= damage;
     }
     
 }