NetworkAnimator.Cleanup() removes OnClientConnectedCallback only if IsServer is true, but ShutdownInternal() sets it to false beforehand.

[davidyrgilbert]

Description

When shutting down a server/host and starting it again with a NetworkAnimator on e.g. the player, an error gets thrown:

NullReferenceException: Object reference not set to an instance of an object
Unity.Netcode.Components.NetworkAnimator.OnClientConnectedCallback (System.UInt64 playerId) (at Library/PackageCache/com.unity.netcode.gameobjects@1.0.0/Components/NetworkAnimator.cs:447)
Unity.Netcode.NetworkManager.InvokeOnClientConnectedCallback (System.UInt64 clientId) (at Library/PackageCache/com.unity.netcode.gameobjects@1.0.0/Runtime/Core/NetworkManager.cs:379)
Unity.Netcode.NetworkManager.HandleConnectionApproval (System.UInt64 ownerClientId, Unity.Netcode.NetworkManager+ConnectionApprovalResponse response) (at Library/PackageCache/com.unity.netcode.gameobjects@1.0.0/Runtime/Core/NetworkManager.cs:2077)
Unity.Netcode.NetworkManager.StartHost () (at Library/PackageCache/com.unity.netcode.gameobjects@1.0.0/Runtime/Core/NetworkManager.cs:1126)

It's pretty clear why that happens:

For the NetworkAnimator, OnNetworkSpawn:

                if (IsServer)
                {
                    NetworkManager.OnClientConnectedCallback += OnClientConnectedCallback;
                }

Now, on Cleanup(), the callback is removed again:

        private void Cleanup()
        {
            ...
            if (IsServer)
            {
                NetworkManager.OnClientConnectedCallback -= OnClientConnectedCallback;
            }
            ...

However, cleanup gets called here:

internal void ShutdownInternal()
{
            ....
            IsConnectedClient = false;
            IsServer = false;
            IsClient = false;

            this.UnregisterAllNetworkUpdates();

            ....            

            if (SpawnManager != null)
            {
                SpawnManager.DespawnAndDestroyNetworkObjects();            <---- this is where Cleanup gets called
                SpawnManager.ServerResetShudownStateForSceneObjects();

                SpawnManager = null;
            }
}

Then, the callback is not being cleaned up, but the object behind it is deleted. Connecting again then tries to execute that cleanup but fails, obviously.

### Reproduce Steps

1. Add NetworkAnimator to e.g. PlayerPrefab
2. StartHost()
3. StopHost()
4. StartHost() again. 

Not 100% sure this is enough as we're doing a few initialization things inbetween, but the issue should be quite clear from the code.

### Actual Outcome

Callback doesn't get removed

### Expected Outcome

Callback gets removed

### Screenshots

If applicable, add screenshots to help explain your problem.

### Environment

- OS: Any
- Unity Version: 2021.3.5
- Netcode Version: 1.0.0
- Netcode Commit: https://github.com/Unity-Technologies/com.unity.netcode.gameobjects/commit/39a244854e323e5e0f444bc17bb7662f66ce28f2#diff-d4e79dceee7c1288ed9747284d2b61057e03cc974cfb21af8afb2704b1fada99R258

### Additional Context

[ashwinimurt]

@NoelStephensUnity I think this belongs to you as well :)

[ashwinimurt]

Backlog MTT-4385

[NoelStephensUnity]

Hi @davidyrgilbert!
I am closing this out as it was fixed in PR-2074 and the fix will be included in the next update. If this is blocking your progress, you can always use the develop branch that has the most recent fixes applied to it until the package is updated.