Loading a Texture2D using the inspector, what am I doing wrong?

Hello, I'm trying load a png file into one of my scripts. In my script file, I have the following...

 using UnityEngine;
 using System.Collections;
 
 public class RenderPlaceholder : MonoBehaviour {
     
     public Texture2D hexGridOverlayTexture;
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     void OnGUI ()
     {        
         GUI.DrawTexture(new Rect(10, 10, 200, 100), hexGridOverlayTexture, ScaleMode.ScaleToFit, true, 0f);
 .
 .
 .

Then, I dragged the .png file into a "textures" folder in my Project view. Then, I selected the script in the Project view and dragged the new texture from my "textures" folder onto the texture attribute in the Inspector view. That texture attribute shows that is it set to my .png image. Wonderful.

When I run my script, I get the following error...

"NullReferenceException: Object reference not set to an instance of an object"

What am I doing wrong?

Thanks so much for your wisdom!