Unity Netcode - Method not being called

I have a Network Object to hold all of the player's weapons, w$$anonymous$$ch needs to be spawned in seperately as a prefab. I then need to spawn in a starting weapon for that player straight after t$$anonymous$$s, w$$anonymous$$ch is again a Network Object. These need to be seperate methods though, because the same logic to add a weapon can be called during the game when the player picks up a new weapon.

When I run t$$anonymous$$s code, it creates the initial Gun Holder Objects as expected, and the Host/Server player also has their first gun added to the Holder object. But when the client joins, it creates the Gun Holder, but then skips through the UpdateGuns method w$$anonymous$$ch allocates the initial gun prefab, but then still seems to run the second AddNewGun_ServerRpc method, because at that point it throws a null pointer saying that the currentNewGun hasn't been assigned. I've checked t$$anonymous$$s with logging, where on the second player joining it skips logging for UpdateGuns but still logs for AddNewGun_ServerRpc.

Does anyone know what I'm doing wrong here? It must be somet$$anonymous$$ng to do with the order I'm calling the methods combined with where the calls are being run (Server or Client), but I can't wrap my head around t$$anonymous$$s.

 public Transform gunHolder;
 
 public GameObject gunHolderNetworkObject;
 private GameObject gunHolderReference;
 
 public GameObject startGun;
 public GameObject currentNewGun;
 
 private void Start()
     {
         if (!IsOwner) return;
 
         SetUpGuns_ServerRpc();
 
         UpdateGuns(startGun);
     }
 
 public void UpdateGuns(GameObject gun)
     {
         if (!IsOwner) return;
 
         currentNewGun = gun;
 
         AddNewGun_ServerRpc();
     }
 
 [ServerRpc(RequireOwners$$anonymous$$p = false)]
     private void SetUpGuns_ServerRpc(ServerRpcParams serverRpcParams = default)
     {
         var clientId = serverRpcParams.Receive.SenderClientId;
         if (NetworkManager.ConnectedClients.ContainsKey(clientId))
         {
             NetworkClient client = NetworkManager.ConnectedClients[clientId];
             Transform clientGunHolder = client.PlayerObject.GetComponent<PlayerController>().gunHolder;
 
             gunHolderReference = Instantiate(gunHolderNetworkObject, clientGunHolder.position, clientGunHolder.rotation);
             gunHolderReference.GetComponent<NetworkObject>().SpawnWithOwners$$anonymous$$p(clientId);
             gunHolderReference.GetComponent<GunHolderReference>().SetGunHolder(clientGunHolder.gameObject);
         }
     }
 
 [ServerRpc(RequireOwners$$anonymous$$p = false)]
     private void AddNewGun_ServerRpc(ServerRpcParams serverRpcParams = default)
     {
         var clientId = serverRpcParams.Receive.SenderClientId;
         if (NetworkManager.ConnectedClients.ContainsKey(clientId))
         {
             NetworkClient client = NetworkManager.ConnectedClients[clientId];
             GameObject currentNewGun = client.PlayerObject.GetComponent<PlayerController>().currentNewGun;
 
             GameObject newGun = Instantiate(currentNewGun, gunHolder.position, gunHolder.rotation);
             newGun.GetComponent<NetworkObject>().SpawnWithOwners$$anonymous$$p(clientId);
 
             newGun.transform.parent = gunHolderReference.transform;
 
             playerGuns.Add(newGun.GetComponent<Gun>());
             newGun.GetComponent<Gun>().currentAmmo = newGun.GetComponent<Gun>().maxAmmo;
 
             UIController.instance.ShowNotification($"{Gun.gunNames[newGun.GetComponent<Gun>().gunType]}");
         }
     }