Null reference for : SceneManager.CheckForAndSendNetworkObjectSceneChanged(); in NetworkManager Class

[amirhoseynfatemi]

### Description

I made an online game with NGO and it shows me a lot of null reference errors.

And this error only happens in Android and is not a problem in Unity!

With Android Logcat, I found out that the error is in the Unity NetworkManager script and from this part(Screenshots).

In Android, when the player enters the scene where the NetworkObject is located
In the scene, I have a script to connect to the server, this script works well in Unity
But such a problem occurs in Android
I have no idea where the problem is!!!

Can anyone help me, what is the cause of this error? Thank you.

Screenshots

Environment

• OS: Android
• Unity Version: 6.0.32f1
• Netcode Version: 2.2.0

[NoelStephensUnity]

That error means that for some reason the NetworkManager is no longer valid.
Could you provide us with a description of what events occur up to getting this error (i.e. describe steps taken from starting a game session up to the point of the error happening)?

[amirhoseynfatemi]

Hi @NoelStephensUnity
Thanks for Reply

I have a scene that contains a NetworkManager as well as a GameObject with a script for connecting a client to a dedicated server.

In the Start method of this script, I subscribe to OnConnectionEvent and OnTransportFailure.
Then, I specify the IP and port using SetConnectionData, and I call StartClient in the next line.

In the NetworkManager, I use a prefab named HeroPresent as the Default Player Prefab. When this prefab is spawned, it triggers the OnNetworkSpawn method, where a ServerRpc informs the server which player should be spawned.
Calling the ServerRpc method can only be done by the Owner and not the Server, as shown below:

public override void OnNetworkSpawn()
{
    if (!IsOwner || IsServer) return;
    // .......
}

The server then instantiates the desired player, accesses its NetworkObject, and spawns it using SpawnAsPlayerObject.

Each time this process completes, the server checks how many players have spawned in the game. If the number of spawned players reaches the desired count, the server starts the game.
At this point, a ClientRpc notifies all connected players that the game has started.

The method OnConnectionEvent, which is subscribed to OnConnectionEvent, is implemented as follows:

private void OnConnectionEvent(NetworkManager arg1, ConnectionEventData arg2)
    {
        switch (arg2.EventType)
        {
            case ConnectionEvent.ClientConnected:
                waitPageText.text = "Waiting for an opponent to join!";
                break;

            case ConnectionEvent.PeerConnected:
                waitPage.gameObject.SetActive(false);
                MessageShower.Instance.Show("Your opponent has joined the game");
                break;
            case ConnectionEvent.ClientDisconnected:
                
                break;
            case ConnectionEvent.PeerDisconnected:
                    MessageShower.Instance.Show("Your opponent has left the game");
                break;
        }
    }

Additionally, there is a method named CancelGame, which is subscribed to OnTransportFailure. This method simply loads another scene in Single mode.

This is the entire process I follow. However, I encounter errors on Android, even though everything works properly in the Unity Editor.

[amirhoseynfatemi]

@NoelStephensUnity

Hello again,

So, I found the problem.
The issue was with how the scene was being loaded. I was loading the scenes I had using Addressable. When I loaded them using SceneManager, everything worked correctly on Android too. However, I couldn't understand why this problem occurs when using Addressable. Should I clear the Addressable cache before loading the scene?

- The code for loading the scene using Addressable:

var asyncOperation = Addressables.LoadSceneAsync(sceneName, LoadSceneMode.Single);

            // Wait until the scene is loaded
            yield return asyncOperation;

            if (asyncOperation.Status != AsyncOperationStatus.Succeeded)
            {
                Debug.LogError("Failed to load scene");
                yield break;
            }

- The code for loading the scene using SceneManager:

var asyncOperation = SceneManager.LoadSceneAsync(scene.ToString());

        while (asyncOperation is { isDone: false })
            yield return null;


        if (asyncOperation is not { isDone: true }) yield break;

[NoelStephensUnity]

Hi @amirhoseynfatemi,

Ahhh...I understand now. So, if the addressable scene is not in the project's scenes in build list then this will be an issue as it will fail to properly synchronize and any netcode related assets within the scene won't be properly spawned on clients joining a session. The symptom should look like:

• If you host a session on the Android, it could appear to load just fine.
• If an Android client attempts to join, then the client will have issues as it won't know how to synchronize the addressable scene(s).

Currently, NGO only supports using addressable network prefabs.
You could have:
A scene that is added to the scenes in build list. That scene could contain an in-scene placed network prefab that that has a NetworkBehaviour which could contain a list of addressable network prefabs to load.

Now, if the addressable scene contains no netcode assets and is purely just visual related assets like terrain, models/geometry, environmental sound, music, and such... then that is perfectly fine.

Look at this example to see how you can pre-load scenes.

In the above example you will want to look at:

• SceneBootstrapLoader.cs
  • This handles starting a host, client, server, or dedicated server and the scene loading side of things.
• NetworkManagerBootstrapper.cs
  • This has a reference to the SceneBootstrapLoader and invokes its StartSession method that kicks off a coroutine.

This will handle the loading "local" scenes portion. However, if you want to dynamically change what "local scene" is loaded then you will need to modify the above example a bit with something similar to the SceneBootstrapLoader with the adjustments (InGameAddressableSceneLoader):

• It has to be a NetworkBehaviour and that would be placed on an in-scene placed NetworkObject in a scene that is pre-defined/local to the project (and included in the scenes in build list).
• It will need to have an additional status for newly joining clients where:
  • Upon the client finishing the typical synchronization, the InGameAddressableSceneLoader would override the OnSynchronizeComplete.

Here is some pseudo code that shows a general direction of what I am describing:

public class PlayerAddressableSceneLoaderStatus : NetworkBehaviour
{
    public enum PlayerSceneLoaderStatus
    {
        None,
        LoadingScenes,
        ScenesLoaded
    }

    private NetworkVariable<PlayerSceneLoaderStatus> m_PlayerLoaderState =
        new NetworkVariable<PlayerSceneLoaderStatus>(default,
            NetworkVariableReadPermission.Everyone, NetworkVariableWritePermission.Owner);

    public void SetPlayerSceneLoaderStatus(PlayerSceneLoaderStatus playerSceneLoaderStatus)
    {
        m_PlayerLoaderState.Value  = playerSceneLoaderStatus;
    }

    public override void OnNetworkDespawn()
    {
        if (IsServer && !IsOwner)
        {
            m_PlayerLoaderState.OnValueChanged += OnPlayerLoaderStateChanged;
        }
        base.OnNetworkDespawn();
    }

    private void OnPlayerLoaderStateChanged(PlayerSceneLoaderStatus previous, PlayerSceneLoaderStatus current)
    {
        if (current == PlayerSceneLoaderStatus.ScenesLoaded)
        {
            // Server knows the player has finished loading the addressable scenes
        }
    }
}

public class InGameAddressableSceneLoader : NetworkBehaviour
{
    private PlayerAddressableSceneLoaderStatus m_PlayerLoaderStatus;

    protected override void OnNetworkSessionSynchronized()
    {
        m_PlayerLoaderStatus = NetworkManager.LocalClient.PlayerObject.GetComponent<PlayerAddressableSceneLoaderStatus>();
        // Start loading the addressable scene
        base.OnNetworkSessionSynchronized();

    }

    private IEnumerator AddressableSceneLoading()
    {
        // Start loading your addressable scenes
        m_PlayerLoaderStatus.SetPlayerSceneLoaderStatus(PlayerAddressableSceneLoaderStatus.PlayerSceneLoaderStatus.LoadingScenes);
        var continueLoading = true;
            
        while(continueLoading)
        {
            // loop until you are done loading scenes

            yield return null;
        }
        // Notify the client is done loading the addressable scenes
        m_PlayerLoaderStatus.SetPlayerSceneLoaderStatus(PlayerAddressableSceneLoaderStatus.PlayerSceneLoaderStatus.ScenesLoaded);
    }
}