How can I use a GameObject to hold all references in a network game?

I have a GameObject called AppManager and I want to hold all the references to the different GameObjects in my game to make them easily accessible from other scripts. Like this:

 using UnityEngine;
 using UnityEngine.Networking;
 
 public class AppManager : NetworkBehaviour
 {
     [HideInInspector]
     public static AppManager instance = null;
 
     private HudController hudController;
 
     public override void OnStartLocalPlayer()
     {
         base.OnStartLocalPlayer();
 
         if (instance == null)
         {
             instance = this;
         }
         else if (instance != this)
         {
             Destroy(gameObject);
         }
 
         Debug.Log("OnStartLocalPlayer() " + GameObject.FindGameObjectWithTag("HUD"));
 
         this.hudController = GameObject.FindGameObjectWithTag("HUD").GetComponent<HudController>();
     }
 
     // Getter
     public HudController GetHudController()
     {
         return this.hudController;
     }
 }
 

Then I try to access the AppManager in another script called GameTime:

  [ClientRpc]
 public void RpcUpdateGameTimeOnClients(int quarterHours, int gameDay)
 {
     string gameTimeText;
 
     // do some stuff to get a proper string...
 
     // this line throws a NullReferenceException
     AppManager.instance.GetHudController().UpdateGameTime(gameTimeText);
     
     // but when I access the components directly and do
     // the same stuff like the UpdateGameTime() method it works
     GameObject.FindGameObjectWithTag("GameTimeText").GetComponent<Text>().text = gameTimeText;
     Debug.Log(GameObject.FindGameObjectWithTag("GameManager"));
 }

So I wonder why I can't get an instance of AppManager in my GameTime script? There should be an instance of AppManager on the server as well. I didn't check neither "Server Only" nor "Local Layer Authority" in the NetworkIdentity of AppManager.