Missing Reference Exception when changing scene while player object is destroyed

[Ed-Gilmour]

When I load the players into a game and then destroy a client player object and have them leave while their player object is destroyed there is a missing reference exception that endlessly runs for the client the rest of time they have the game open.

MissingReferenceException: The object of type 'Unity.Netcode.NetworkObject' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object. UnityEngine.Object+MarshalledUnityObject.TryThrowEditorNullExceptionObject (UnityEngine.Object unityObj, System.String parameterName) (at <84f0d810adef4e6c8deab33e4ae93f7c>:0) UnityEngine.Bindings.ThrowHelper.ThrowNullReferenceException (System.Object obj) (at <84f0d810adef4e6c8deab33e4ae93f7c>:0) UnityEngine.Component.get_gameObject () (at <84f0d810adef4e6c8deab33e4ae93f7c>:0) Unity.Netcode.NetworkObject.UpdateForSceneChanges () (at ./Library/PackageCache/com.unity.netcode.gameobjects@f2477a6c90fe/Runtime/Core/NetworkObject.cs:3318) Unity.Netcode.NetworkObject.UpdateNetworkObjectSceneChanges () (at ./Library/PackageCache/com.unity.netcode.gameobjects@f2477a6c90fe/Runtime/Core/NetworkObject.cs:3287) Unity.Netcode.NetworkManager.NetworkUpdate (Unity.Netcode.NetworkUpdateStage updateStage) (at ./Library/PackageCache/com.unity.netcode.gameobjects@f2477a6c90fe/Runtime/Core/NetworkManager.cs:404) Unity.Netcode.NetworkUpdateLoop.RunNetworkUpdateStage (Unity.Netcode.NetworkUpdateStage updateStage) (at ./Library/PackageCache/com.unity.netcode.gameobjects@f2477a6c90fe/Runtime/Core/NetworkUpdateLoop.cs:191) Unity.Netcode.NetworkUpdateLoop+NetworkPostLateUpdate+<>c.<CreateLoopSystem>b__0_0 () (at ./Library/PackageCache/com.unity.netcode.gameobjects@f2477a6c90fe/Runtime/Core/NetworkUpdateLoop.cs:286)

1. Spawn player object on a client
2. Despawn client player object
3. While client player object is despawned load a different scene
4. See error on client every frame

Error on client runs every frame for the rest of the time while the game is open. I tried going into the NetworkObject script and adding if(this == null) return; on line 3315 (UpdateForSceneChanges) and it stops making the error. This makes me believe that it might be getting cached somewhere like the scenemanager and its not getting cleared properly when the object is destroyed on shutdown.

I except the error to not appear and the rest of the game run normally.

• OS: Windows 11
• Unity Version: 6000.0.36f1
• Netcode Version: 2.2.0

The attached game object to the NetworkObject when I was debugging it was null which is why I think it's not getting cleared properly and why the this == null is stopping the error every frame.

[NoelStephensUnity]

@Ed-Gilmour
I am having a hard time replicating this issue. Could you provide additional details as to how you are handling player spawning?

Are you letting the NetworkManager handle it via the player prefab property or are you handling it via your own script?

If you are handling this via your own script, could you provide the code that is spawning and despawining the player prefab?

Also, could you provide a screenshot of your player prefab in the inspector view? I am primarily interested in the NetworkObject's settings.

[Ed-Gilmour]

@NoelStephensUnity
Sure,

I'm spawning the prefab via my own script so here is the code

GameObject playerObj = Instantiate(playerPrefab, spawnTransform.position, spawnTransform.rotation);
NetworkObject playerNetObj = playerObj.GetComponent<NetworkObject>();
playerNetObj.SpawnAsPlayerObject(NetworkManager.LocalClientID, true);

Here's a screenshot of the network object, network transform, and network rigidbody 2d on the player object.

I appreciate you looking into this, let me know if there's anything else I need to show you.

[NoelStephensUnity]

@Ed-Gilmour

Are you using a distributed authority network topology?

[Ed-Gilmour]

@NoelStephensUnity
I'm using Client Server, when Distributed Authority was added it defaulted my network objects to it. Is this the issue? What should I have the network objects be?

[NoelStephensUnity]

No worries... if you install the Multiplayer SDK it exposes those permissions and if you are using a client-server network topology it ignores the permissions.

If you could include the script that despawns the player prefab instance that would also help (whether you are destroying it or not).

Final questions:

• What event/logic causes the player to get despawned?
• In your initial description, you mention the client disconnecting prior to doing the scene migration (scene loading as Single mode). Was this a typo or does the client completely disconnect after its player prefab instance is destroyed?
  • If so, is the order of operations like this?
    • (Event occurs) Client's player prefab instance is despawned by the host/server.
    • Client disconnects.
    • Host transitions to a new scene.
    • (If this is not the order of events could you list those out?)

Much appreciated on the quick responses and providing additional details. It will help me replicate the issue.
If you are able, providing a replication project can expedite things. You would only need to zip up the Assets, Packages, and Project Settings folders (as long as you don't have a bunch of graphics/assets it keeps the file size pretty small).

[Ed-Gilmour]

@NoelStephensUnity
Here's the code that despawns the player network object

        if(IsHost)
        {
            NetworkObject.Despawn();
        }

The player is despawned when their health goes below zero usually when hit by a projectile or when running into something.

After the client is despawned the error happens when opening the pause menu and exiting the game back to the menu from there while their player object is still destroyed (the player object will respawn after a timer so exiting before this causes the error).

As for the order of events the error occurs when the client transitions to a new scene and not the host, so it goes like this:

• Client's player prefab instance is despawned by the host/server
• Client disconnects (due to exiting the game through the pause menu)
• Client transitions to a new scene (the menu)

The project is fairly big so I can try and provide a replication project if that's sill necessary.

[NoelStephensUnity]

Are you recycling the player prefab instance?
In other words, when you despawn the player prefab are you just despawning it and not destroying it?

If so and the client's player prefab is not destroyed but like disabled, then you might mark the player prefab's NetworkObject to not be destroyed with a scene load:

NetworkObject.DestroyWithScene = false;

[Ed-Gilmour]

It is being destroyed when I despawn it, so it's not going into the next scene.

[NoelStephensUnity]

While I haven't narrowed down the issue, for the time being you can disable the "Scene Migration Synchronization" for your player prefab which will prevent it from being added to the dictionary.

[NoelStephensUnity]

@Ed-Gilmour
Other than the exception you are reporting, do you see any exceptions when the client's player prefab instance is despawned =or= are you destroying the associated GameObject?

The only way I can replicate this issue is if the client destroys the player prefab instance.

The player is despawned when their health goes below zero usually when hit by a projectile or when running into something.

You might verify that when this condition happens that there isn't any code that is being invoked on the client side that could cause the client-side player prefab instance to be destroyed.

[NoelStephensUnity]

@Ed-Gilmour
I have put together a project that provides us with a good reference point:

PlayerSpawnExamples.zip

There are runtime UI instructions for spawning, despawning, and then despawning and destroying (the GameObject on the client side). If you just despawn and don't destroy the client's player prefab instance, then you shouldn't run into any issues. If you despawn and then sometime shortly after destroy the client-side prefab instance's GameObject (before the client receives the despawn message from the host) then you will see the exceptions occur.

Here is a video where I run through despawning without destroying the GameObject on the client side with no exceptions and then at the end I despawn and then destroy (locally on the client) the NetworkObject's GameObject:

PlayerSpawnDespawnIssue.mp4

While the exception should be handled more gracefully, if you are destroying the GameObject on the client side (before receiving the message to despawn and destroy from the host) then you would get that exception and you could run into other issues down the road even if the exception was handled more gracefully.

Take a look at the included project and let me know if this is what is happening (or something similar) in your project?