Changing a material using Unity Editor Script

I have the following game object

 MyGameObject
 -MeshRenderer
 ----Materials:
 ----- Size: 1
 ----- Element 0: MyGameObjectMaterial

I want to change the gameobject material using a Unity Editor Script, my script looks similar to this

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 public class BuildingAutomator : EditorWindow
 {
     public GameObject myGameObject;
     public Material myNewMaterial;
     
     // Add menu named "My Window" to the Window menu
     [MenuItem("Window/My Window")]
     static void Init()
     {
         // Get existing open window or if none, make a new one:
         BuildingAutomator window = (BuildingAutomator)EditorWindow.GetWindow(typeof(BuildingAutomator));
     }
 
     void OnGUI()
     {
         if(GUILayout.Button("Change material!"))
         {
             myGameObject.renderer.materials[0] = myNewMaterial; //<--- Error
         }
     }
 }

But it throws the following error:

 Instantiating material due to calling renderer.material during edit mode. This will leak materials into the scene. You most likely want to use renderer.sharedMaterial instead.
 UnityEngine.Renderer:get_materials()

Any suggestions?