KSP toolbar setup not working as expected

ToolBarTest.cs

using System;
using UnityEngine;
using KSP.UI.Screens;

namespace ToolBarTest
{

    [KSPAddon(KSPAddon.Startup.MainMenu, false)]
    public class ToolBarTest : MonoBehaviour
    {
        public static ApplicationLauncherButton button_1;
        public static ApplicationLauncherButton button_2;


        Texture2D test_texture_1 = new Texture2D(1, 1, TextureFormat.RGBA32, false);
        Texture2D test_texture_2 = new Texture2D(1, 1, TextureFormat.RGBA32, false);

        private void Awake(){
            test_texture_1.SetPixel(0, 0, Color.blue);
            test_texture_1.Apply();
            test_texture_2.SetPixel(0, 0, Color.red);
            test_texture_2.Apply();

            Debug.Log ("Adding App launcher event callbacks");
            GameEvents.onGUIApplicationLauncherReady.Add (this.app_launcher_ready);
            GameEvents.onGUIApplicationLauncherDestroyed.Add (this.app_launcher_destroyed);
        }
        
        public void app_launcher_ready(){
            GameEvents.onGUIApplicationLauncherReady.Remove (this.app_launcher_ready);
            Debug.Log ("app launcher is ready");
            add_to_toolbar ();
        }
        
        public void app_launcher_destroyed(){
            Debug.Log ("app launcher destroyed");
            remove_from_toolbar();
        }
        
        public void add_to_toolbar(){
            Debug.Log ("Adding buttons to toolbar");
            ToolBarTest.button_1 = ApplicationLauncher.Instance.AddModApplication (
                button_1_action, button_1_action,
                null, null,
                null, null,
                ApplicationLauncher.AppScenes.VAB | ApplicationLauncher.AppScenes.SPH,
                test_texture_1
            );

            ToolBarTest.button_2 = ApplicationLauncher.Instance.AddModApplication (
                button_2_action, button_2_action,
                null, null,
                null, null,
                ApplicationLauncher.AppScenes.SPACECENTER,
                test_texture_2
            );
        }

        public void remove_from_toolbar(){
            Debug.Log ("removing button from toolbar");
            ApplicationLauncher.Instance.RemoveModApplication (button_1);
            ApplicationLauncher.Instance.RemoveModApplication (button_2);
        }
        
        public void button_1_action(){
            Debug.Log ("button 1: Oi! someone clicked me.");
        }

        public void button_2_action(){
            Debug.Log ("button 2: quit poking me.");
        }
    }
}